rootfs.py: allow removal of unneeded packages

rootfs.py: allow removal of unneeded packages

Current functionality allows for the removal of certain packages
based on the read-only image feature. This patch extends this
functionality by adding the FORCE_RO_REMOVE variable, which will
remove these packages regardless of any image features.

[ YOCTO #9491 ]

(From OE-Core rev: cfb869ffd4c37c3cc8e6b3eb732c1a7b7cfc3cb0)

Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>