(image|rootfs_ipk).bbclass, rootfs.py: ipk multilib fixes

This commit whitelists some common directories, so the multilib sanity
checks pass and also fixes an issue in lib/oe/rootfs.py when the
compared files do not exist.

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>