Force the postinst call in factory_Wipe to NOT update firmware

This path is supposed to update nothing at that stage.
In order to prevent confusion, we explicitly disable it.

BUG=chromium-os:6512
TEST=manually:
1. build image with firmware updates enabled by default.
2. run Wipe test.
3. firmware should not updated.

Change-Id: Id4f1a25b3c8ffb0b2a0270c272312813c2fab61a

Review URL: http://codereview.chromium.org/3302013
1 file changed