Read fstab from the source build.

When creating incremental OTAs, we should always use the fstab from the
source build instead of the target.

Bug: 21948974
Change-Id: I8b6a18f64b412f782184f9d382544ed0ce093fb4
(cherry picked from commit dd24da9ec91b74b7e3c8d1af9f1f9f792d41ac4d)
1 file changed