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
