Ninja can't depend on environment variables, so modifying build
behavior based on environment variables requires coordinating
between the soong script that invokes ninja and the soong_build
manifest generator.
Allow any module to call Config.Getenv to get the contents of an
environment variable while registering a dependency on it.
After all modules have been processed write out the state of
all used environment variables to a JSON file called
.soong.environment. During the next build the soong script
will use the soong_env tool to compare the contents of
.soong.environment to the current environment, and force a
build manifest regeneration by deleting the .soong.environment
file if any variables have changed.
Change-Id: Id0d81933a857bc2fc1cd7a393a3c6cec73dc4824
bootstrap.bash creates a soong script in the output directory using
build/soong/soong.in. This requires a manual rebootstrap any time
soong.in changes. Instead, have bootstrap.bash symlink
build/soong/soong.bash to soong in the output directory, and create
a file called .soong.bootstrap in the output directory that contains
the variables that bootstrap.bash sets.
Change-Id: I5e6e54c2e8bdde876941e2e082f9ba177c757cbf
