mount-encrypted: allow stateful mount check to pass factory install
mount-encrypted: allow stateful mount check to pass factory install

When factory install happens, mount-encrypted is running on a tmpfs,
which can be detected via a W_OK check on the root filesystem.

BUG=chrome-os-partner:12033
TEST=alex build, manual test

Change-Id: I7bf5eaa244a50dd2a0de51760c964e970fa8e3aa
Signed-off-by: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/28960
Reviewed-by: Gaurav Shah <2987db6a10bcfe664554a372d01e80df7fd325a3@chromium.org>

geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,marineam/seismograph,acorn-marvell/vboot_reference,ccaapton/vboot_reference,marineam/seismograph,ionave/seismograph,marineam/seismograph,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,coreboot/vboot,coreboot/vboot,ccaapton/vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,ccaapton/vboot_reference,ionave/seismograph,ionave/seismograph,coreboot/vboot,marineam/seismograph