coreboot: make lb_framebuffer a weak function

coreboot: make lb_framebuffer a weak function

This is to support other gfx enable method such as Gfx Peim (AKA GOP)
for Intel soc.

BRANCH=none
BUG=chrome-os-partner:44559
TEST=Built and boot on kunimitsu/glados.

Change-Id: Ib8010ea6901ea906a8b4129807b94ace71ef1165
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: ad26a99560009c487070cccf6ab132188b9e247d
Original-Change-Id: Id132718a8bcec5446cc4c0d9d636d26e8a99bb15
Original-Signed-off-by: robbie zhang <robbie.zhang@intel.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/303801
Original-Commit-Ready: Robbie Zhang <robbie.zhang@intel.com>
Original-Tested-by: Robbie Zhang <robbie.zhang@intel.com>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/12140
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>