factory_install: force mounting with ext2 before running postinst

ext4 mounts on the ext2 fs can cause a superblock write w/ our existing clients.

BUG=chrome-os-partner:3940,chromium-os:11509
TEST=none, next factory bundle will verify

Change-Id: I4cf13c6ba36ee7056c2a79d707149dda3cb5b07d
Reviewed-on: http://gerrit.chromium.org/gerrit/5572
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
(cherry picked from commit ece40f0ab1dd7468c5ecefcaf140a90aea7d74d6)
Reviewed-on: http://gerrit.chromium.org/gerrit/5836
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
1 file changed