imagebuilder: reuse rootfs preparation from rootfs.mk

imagebuilder: reuse rootfs preparation from rootfs.mk

In addition to removing redundant code, this fixes various issues in
IB-generated images that have been fixed in prepare_rootfs before,
including better handling of CONFIG_CLEAN_IPKG and enabling of initscripts
from FILES.

We also reuse the opkg macro and remove --force-... flags that have been
removed from rootfs.mk as well.

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