Allow recovery to create device nodes and modify rootfs

tilapia's OTA code for updating the radio image needs to
create files on rootfs and create a character device in /dev.
Allow it.

(cherrypick from commit de84e45d7e6f9dbd6ba8bfaf592130751d2ea853)

Bug: 18281224
Change-Id: Ic408c2b28e16a40650f71efe2f17fb0c2e71f97f
Merge "Allow recovery to create device nodes and modify rootfs" into lollipop