Replace soong script with a symlink

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
3 files changed