Add doc update for phony targets

# Build System Changes for Android.mk Writers

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

## `.PHONY` rule enforcement  {#phony_targets}

There are several new warnings/errors meant to ensure the proper use of

`.PHONY` targets in order to improve the speed and reliability of incremental

builds.

`.PHONY`-marked targets are often used as shortcuts to provide "friendly" names

for real files to be built, but any target marked with `.PHONY` is also always

considered dirty, needing to be rebuilt every build. This isn't a problem for

aliases or one-off user-requested operations, but if real builds steps depend

on a `.PHONY` target, it can get quite expensive for what should be a tiny

build.

``` make

...mk:42: warning: PHONY target "out/.../foo" looks like a real file (contains a "/")

```

Between this warning and the next, we're requiring that `.PHONY` targets do not

have "/" in them, and real file targets do have a "/". This makes it more

obvious when reading makefiles what is happening, and will help the build

system differentiate these in the future too.

``` make

...mk:42: warning: writing to readonly directory: "kernel-modules"

```

This warning will show up for one of two reasons:

1. The target isn't intended to be a real file, and should be marked with

   `.PHONY`. This would be the case for this example.

2. The target is a real file, but it's outside the output directories. All

   outputs from the build system should be within the output directory,

   otherwise `m clean` is unable to clean the build, and future builds may not

   work properly.

``` make

...mk:42: warning: real file "out/.../foo" depends on PHONY target "buildbins"

```

If the first target isn't intended to be a real file, then it should be marked

with `.PHONY`, which will satisfy this warning. This isn't the case for this

example, as we require `.PHONY` targets not to have '/' in them.

If the second (PHONY) target is a real file, it may unnecessarily be marked

with `.PHONY`.

### `.PHONY` and calling other build systems

One common pattern (mostly outside AOSP) that we've seen hit these warning is

when building with external build systems (firmware, bootloader, kernel, etc).

Those are often marked as `.PHONY` because the Android build system doesn't

have enough dependencies to know when to run the other build system again

during an incremental build.

We recommend to build these outside of Android, and deliver prebuilts into the

Android tree instead of decreasing the speed and reliability of the incremental

Android build.

In cases where that's not desired, to preserve the speed of Android

incrementals, over-specifying dependencies is likely a better option than

marking it with `.PHONY`:

``` make

out/target/.../zImage: $(sort $(shell find -L $(KERNEL_SRCDIR)))

	...

```

For reliability, many of these other build systems do not guarantee the same

level of incremental build assurances as the Android Build is attempting to do

-- without custom checks, Make doesn't rebuild objects when CFLAGS change, etc.

In order to fix this, our recommendation is to do clean builds for each of

these external build systems every time anything they rely on changes. For

relatively smaller builds (like the kernel), this may be reasonable as long as

you're not trying to actively debug the kernel.

## `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.