Refactor option rom initialization code in coreboot.

Refactor option rom initialization code in coreboot.

- move int15 handler out of the generic code into the mainboard directories
  of those mainboards that actually use it.
- move vbe headers to vbe.h
- move function prototypes used in native oprom code to x86.h

Change-Id: Idfff5e804ea328f7b5feebac72497c97329320ee
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/255
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>