rootfs: fail on errors in postinst scripts

The Gluon firmware framework [1] uses postinst scripts for sanity checks.
Make the build fail when a postinst script exits with an error to make
these sanity checks effective.

All postinst scripts in packages from the LEDE core and the packages feed
seem to work correctly with this change and will always return 0 unless
something is very broken.

[1] https://github.com/freifunk-gluon/gluon



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