staging/multilib: Fix manifest corruption

staging/multilib: Fix manifest corruption

The previous fix wasn't enough to address all the possible ways the
manifests might be ordered. Rework the previous fix so it is tied
to the multilib cross-canadian code which is causing the problem.

RECIPE_SYSROOT_MANIFEST_SUBDIR is not documented as I'd hope nobody
ever needs to use this outside the core multilib code.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>