fs_mgr: split fstab parsing into separate file

Move fstab parsing into fs_mgr_fstab.c so that it compiles into a
separate compilation unit.  uncrypt links against it but doesn't
need to pull in the rest of the dependencies on fs_mgr.c.

Change-Id: I6bffe1cd42720151533e958028259d1931aaa4bf
3 files changed