rootdir: make sure the /oem mountpoint is always available

The /oem mount point is used to mount semi-trusted data, and
many Android One devices depend on it. Make sure it's guaranteed
to always be available.

(cherrypicked from commit f3b554fc614fffaa5fc62ef1b4147131a8fa373c)

Bug: 20816563
Change-Id: Ib5272f025d14d4da6125d753879054b3faeae696
1 file changed