rootfs.py: merge changes from updated rootfs.py upstream

The commit 08a38a7865c41ec60f4b993b964f8d477ea0f680 in upstream poky replaces
oe.utils.contains to bb.utils.contains which also affects rootfs.py. So merge
latest changes from upstream rootfs.py

Signed-off-by: Fahad Usman <fahad_usman@mentor.com>
Merge pull request #168 from fahadusman/update-rootfs.py

rootfs.py: merge changes from updated rootfs.py upstream