bootmode: add display_init_required()

Some of the Chrome OS boards were directly calling vboot
called in some form after contorting around #ifdef preprocessor
macros. The reasoning is that Chrome OS doesn't always do display
initialization during startup. It's runtime dependent. While
this is a requirement that doesn't mean vboot functions should be
sprinkled around in the mainboard and chipset code. Instead provide
one function, display_init_required(), that provides the policy
for determining display initialization action. For Chrome OS
devices this function honors vboot_skip_display_init() and all
other configurations default to initializing display.

Change-Id: I403213e22c0e621e148773597a550addfbaf3f7e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/11490
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
9 files changed