base-files: default_postinst: propagate the real postinst return code

Using the postinst script for sanity checks and expecting opkg to fail
if the postinst didn't return 0 was possible in Barrier Breaker, propagate
the real postinst return code through default_postinst to restore this
behaviour.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

SVN-Revision: 46653