Merge changes from topics "deprecate_export", "add_build_broken"

# Build System Changes for Android.mk Writers

### `export` and `unexport` deprecation  {#export_keyword}

The `export` and `unexport` keywords have been deprecated, and will throw

warnings or errors depending on where they are used.

Early in the make system, during product configuration and BoardConfig.mk

reading: these will throw a warnings, and will be an error in the future.

Device specific configuration should not be able to affect common core build

steps -- we're looking at triggering build steps to be invalidated if the set

of environment variables they can access changes. If device specific

configuration is allowed to change those, switching devices with the same

output directory could become significantly more expensive than it already can

be.

Later, during Android.mk files, and later tasks: these will throw errors, since

it is increasingly likely that they are being used incorrectly, attempting to

change the environment for a single build step, and instead setting it for

hundreds of thousands.

It is not recommended to just move the environment variable setting outside of

the build (in vendorsetup.sh, or some other configuration script or wrapper).

We expect to limit the environment variables that the build respects in the

future, others will be cleared. (There will be methods to get custom variables

into the build, just not to every build step)

Instead, write the export commands into the rule command lines themselves:

``` make

$(intermediates)/generated_output.img:

	rm -rf $@

	export MY_ENV_A="$(MY_A)"; make ...

```

If you want to set many environment variables, and/or use them many times,

write them out to a script and source the script:

``` make

envsh := $(intermediates)/env.sh

$(envsh):

	rm -rf $@

	echo 'export MY_ENV_A="$(MY_A)"' >$@

	echo 'export MY_ENV_B="$(MY_B)"' >>$@

$(intermediates)/generated_output.img: PRIVATE_ENV := $(envsh)

$(intermediates)/generated_output.img: $(envsh) a/b/c/package.sh

	rm -rf $@

	source $(PRIVATE_ENV); make ...

	source $(PRIVATE_ENV); a/b/c/package.sh ...

```

## Implicit make rules are deprecated {#implicit_rules}

Implicit rules look something like the following:

ifneq ($(filter-out false,$(USE_CCACHE)),)

  # The default check uses size and modification time, causing false misses

  # since the mtime depends when the repo was checked out

  export CCACHE_COMPILERCHECK ?= content

  CCACHE_COMPILERCHECK ?= content

  # See man page, optimizations to get more cache hits

  # implies that __DATE__ and __TIME__ are not critical for functionality.

  # Ignore include file modification time since it will depend on when

  # the repo was checked out

  export CCACHE_SLOPPINESS := time_macros,include_file_mtime,file_macro

  CCACHE_SLOPPINESS := time_macros,include_file_mtime,file_macro

  # Turn all preprocessor absolute paths into relative paths.

  # Fixes absolute paths in preprocessed source due to use of -g.

  # We don't really use system headers much so the rootdir is

  # fine; ensures these paths are relative for all Android trees

  # on a workstation.

  export CCACHE_BASEDIR := /

  CCACHE_BASEDIR := /

  # Workaround for ccache with clang.

  # See http://petereisentraut.blogspot.com/2011/09/ccache-and-clang-part-2.html

  export CCACHE_CPP2 := true

  CCACHE_CPP2 := true

  ifndef CC_WRAPPER

    CC_WRAPPER := $(CCACHE_EXEC)

$(KATI_obsolete_var PRODUCT_COMPATIBILITY_MATRIX_LEVEL_OVERRIDE,Set FCM Version in device manifest instead. See $(CHANGES_URL)#PRODUCT_COMPATIBILITY_MATRIX_LEVEL_OVERRIDE)

$(KATI_obsolete_var USE_CLANG_PLATFORM_BUILD,Clang is the only supported Android compiler. See $(CHANGES_URL)#USE_CLANG_PLATFORM_BUILD)

# This is marked as obsolete in envsetup.mk after reading the BoardConfig.mk

$(KATI_deprecate_export It is a global setting. See $(CHANGES_URL)#export_keyword)

CHANGES_URL :=

# Used to force goals to build.  Only use for conditionally defined goals.

# ###############################################################

# Broken build defaults

# ###############################################################

# Assume that all boards have duplicate rules right now.

BUILD_BROKEN_DUP_RULES := true

BUILD_BROKEN_ANDROIDMK_EXPORTS :=

BUILD_BROKEN_DUP_COPY_HEADERS :=

BUILD_BROKEN_DUP_RULES :=

BUILD_BROKEN_PHONY_TARGETS :=

# ###############################################################

# Include sub-configuration files

ifeq ($(CALLED_FROM_SETUP),true)

include $(BUILD_SYSTEM)/ccache.mk

include $(BUILD_SYSTEM)/goma.mk

export CC_WRAPPER

export CXX_WRAPPER

export JAVAC_WRAPPER

endif

ifdef TARGET_PREFER_32_BIT

endif

board_config_mk :=

###########################################

# Handle BUILD_BROKEN_* settings

vars := \

  BUILD_BROKEN_ANDROIDMK_EXPORTS \

  BUILD_BROKEN_DUP_COPY_HEADERS \

  BUILD_BROKEN_DUP_RULES \

  BUILD_BROKEN_PHONY_TARGETS

$(foreach var,$(vars),$(eval $(var) := $$(strip $$($(var)))))

$(foreach var,$(vars), \

  $(if $(filter-out true false,$($(var))), \

    $(error Valid values of $(var) are "true", "false", and "". Not "$($(var))")))

.KATI_READONLY := $(vars)

CHANGES_URL := https://android.googlesource.com/platform/build/+/master/Changes.md

# "" is equivalent to true currently.

ifeq ($(BUILD_BROKEN_ANDROIDMK_EXPORTS),false)

$(KATI_obsolete_export It is a global setting. See $(CHANGES_URL)#export_keyword)

endif

CHANGES_URL :=

###########################################

# Now we can substitute with the real value of TARGET_COPY_OUT_VENDOR

ifeq ($(TARGET_COPY_OUT_VENDOR),$(_vendor_path_placeholder))

# limitations under the License.

#

# Used by the compiler wrapper, but should only be set by gomacc

unexport GOMACC_PATH

# Notice: this works only with Google's Goma build infrastructure.

ifneq ($(filter-out false,$(USE_GOMA)),)

  # Goma requires a lot of processes and file descriptors.