package-release.sh: Force remote darwin build to use prebuilt toolchain.

It turns out that performing a remote Darwin build with:

  distrib/package-release.sh --darwin-ssh=<host> --system=darwin

Resulted in broken executables that never loaded correctly, but ended
up with a 'Trace/BPT trap 5' error message, indicating a program in
the dynamic linker.

I could not find a solution for this, but it comes from the fact that
the remote build uses the host toolchain (i.e. 'gcc') to rebuild all
sources from scratch (after they've been copied as a tarball to the
remote host and unpacked under /tmp/).

This patch fixes the issue by forcing the remote build to use the
prebuilt AOSP toolchain, which generates good executables. This means
that the remote machine *must* have a checkout of $AOSP/prebuilts/
somewhere. The default location expected by the script is:

   /Volumes/Android/repo/android/prebuilts/

But this can be changed with --darwin-prebuilts-dir=<path>.
Another new option to the script is --darwin-no-prebuilts, which
will perform a 'broken' build if one wants to try to diagnose/understand
the problem better.

Note that this doesn't happen when performing a normal build under
$AOSP/external/qemu on a Darwin machine, because the prebuilts toolchain
is picked automatically by android-configure.sh in this case.

+ Add --prebuilts-dir=<path> option to android-configure.sh

BUG=18234905

Change-Id: Ib484bebe28fec36e5affcbe068913ab3997b1db4
Merge "package-release.sh: Force remote darwin build to use prebuilt toolchain."