Handle symlinked OUT_DIRs (again)

My previous attempt at properly handling symlinked OUT_DIRs only worked
if the symlink was to a different directory, not one in the same
directory.

This time, make sure that both make and soong use the same
representation of the output directory by passing BUILDDIR to
bootstrap.bash. Soong has been updated to pick whether to use a relative
or absolute path back to the source tree depending on what makes sense.
If the BUILDDIR or the path back to the source tree change, re-run the
bootstrap.

Also, move the $OUT_DIR/Android.mk and $OUT_DIR/CleanSpec.mk generation
to before Kati runs. In the case that $TOP/out was symlinked to
$TOP/out.angler:

 1) Soong generates out.angler/soong/Android.mk
 2) Kati's find generator produces in-memory tree of the filesystem
 3) $(shell ) creates out.angler/Android.mk
 4) The emulated findleaves Android.mk search finds
    out.angler/soong/Android.mk since the in-memory tree is never updated.

This doesn't happen in the normal case, because we pass --prune=$OUT_DIR
to findleaves.

Change-Id: Ib0fdae2e80f75ddcf33a3c8c5ea0978f5308b437