detachables: Print error if enable normal mode when FORCE_DEV on
detachables: Print error if enable normal mode when FORCE_DEV on

Print a debug error and beep when user tries to enable normal mode
when FORCE_DEV gbb flag is enabled.

BUG=b:67828898
BRANCH=None
TEST=Boot up with gbb flag 0x8 enabled. Select "Enabled OS
     Verification" and select confirm.  Should see error message
     printed and a beep.  Confirm switching to normal mode
     screen will not appear.

Change-Id: Ic02558eb4a86555cebc9c1cd6972d0f0600b4ff1
Signed-off-by: Shelley Chen <ab7b8cc85923339aed379eb1c5cc4dccfd2eb3d7@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/730415
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Julius Werner <4d12f06cff3725783b352157673a720bc67c4afe@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot