Soong's multi-architecture building has grown complex, with the
combination of HostOrDevice+HostType+Arch necessary to determine how to
build a variant of a module, and three separate mutators to split each
into its variations.
Combine HostOrDevice+HostType into Os, which will be Linux, Darwin,
Windows, or Android. Store Os+Arch as a single Target.
Change-Id: Iae677eff61a851b65a7192a47f2dc17c1abb4160
And install the tools into a more obvious location. soong_env is not
moved, since we need it to exist early, so that we can use it in
soong.bash in case there's a build failure.
Change-Id: I9bd1fa320d84d180b2cf3deb90782d380666f7a6