installer: force updating firmware in factory install mode.

Factory test images (and install shims) are configured with
"leave_firmware_alone" because they may contain firmware blobs that are
different from the real version to install.

So we have to explicitly assign the "force" flag when factory installer is
trying to update the firmware to the assigned version (by make_factory_package
--firmwareupdate).

BUG=chrome-os-partner:3152
TEST=none, will be updated before commiting

Change-Id: I449de11cb1785fd69890527313fbdce18f8b835f

R=wfrichar@chromium.org,rspangler@chromium.org

Review URL: http://codereview.chromium.org/6795002

chromeos-postinst

diff --git a/chromeos-postinst b/chromeos-postinst
index bdecbd2..ef3d396 100755
--- a/chromeos-postinst
+++ b/chromeos-postinst
@@ -104,8 +104,11 @@
        -x "${FIRMWARE_UPDATE_SCRIPT}" ]; then
     FIRMWARE_UPDATE_MODE=""
     if [ -n "${IS_FACTORY_INSTALL}" ]; then
-      # factory-mode
-      FIRMWARE_UPDATE_MODE="--mode=factory_install"
+      # factory-mode (--force must be the first parameter when being invoked)
+      # Factory install shim will invoke similiar command in memento_updater.sh,
+      # so if you modify any parameters here, please remember to also update
+      # memento_updater.sh.
+      FIRMWARE_UPDATE_MODE="--force --mode=factory_install"
     elif [ -n "${IS_RECOVERY_INSTALL}" ]; then
       # recovery-mode
       FIRMWARE_UPDATE_MODE="--mode=recovery"