[ChromeOS] Don't initialize VGA Option ROM in normal mode

ChromeOS features two different modes: normal mode and developer mode
(aka jailbreak mode). In developer mode, we need to display a warning
screen for security reasons.

However, in normal mode we want to boot blazingly fast. Therefore we
don't run (VGA) option ROMs, unless we have to print something on the
screen before the kernel is loaded.

Change-Id: I37f63d0b082a48e037e65bde2b380f9b8743ed29
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/829
Tested-by: build bot (Jenkins)
Reviewed-by: Mathias Krause <minipli@googlemail.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>