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>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46653 3c298f89-4303-0410-b956-a3cf2f4a3e73