Merge "Add documentation around PATH restrictions"

# Build System Changes for Android.mk Writers

## PATH Tools {#PATH_Tools}

The build has started restricting the external host tools usable inside the

build. This will help ensure that build results are reproducible across

different machines, and catch mistakes before they become larger issues.

To start with, this includes replacing the $PATH with our own directory of

tools, mirroring that of the host PATH.  The only difference so far is the

removal of the host GCC tools. Anything that is not explicitly in the

configuration as allowed will continue functioning, but will generate a log

message. This is expected to become more restrictive over time.

The configuration is located in build/soong/ui/build/paths/config.go, and

contains all the common tools in use in many builds. Anything not in that list

will currently print a warning in the `$OUT_DIR/soong.log` file, including the

command and arguments used, and the process tree in order to help locate the

usage.

In order to fix any issues brought up by these checks, the best way to fix them

is to use tools checked into the tree -- either as prebuilts, or building them

as host tools during the build.

As a temporary measure, you can set `TEMPORARY_DISABLE_PATH_RESTRICTIONS=true`

in your environment to temporarily turn off the error checks and allow any tool

to be used (with logging). Beware that GCC didn't work well with the interposer

used for logging, so this may not help in all cases.

## Deprecating / obsoleting envsetup.sh variables in Makefiles

It is not required to source envsetup.sh before running a build. Many scripts,