device/Kconfig: Put gfx init methods into a choice

Provide all gfx init methods as a Kconfig `choice`. This elimates the
option to select native gfx init along with running a Video BIOS. It's
been only theoretically useful in one corner case: Hybrid graphics
where only one controller is supported by native gfx init. Though I
suppose in that case it's fair to assume that one would use SeaBIOS to
run the VBIOS.

For the case that we want the payload to initialize graphics or no
pre-boot graphics at all, the new symbol NO_GFX_INIT was added to the
choice. If multiple options are available, the default is chosen as
follows:

  * NO_GFX_INIT, if we add a Video BIOS and the payload is SeaBIOS,
  * VGA_ROM_RUN, if we add a Video BIOS and the payload is not SeaBIOS,
  * NATIVE_VGA_INIT, if we don't add a Video BIOS.

As a side effect, libgfxinit is now an independent choice.

Change-Id: I06bc65ecf3724f299f59888a97219fdbd3d2d08b
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/19814
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/slippy/Kconfig

@@ -15,7 +15,6 @@ config BOARD_GOOGLE_BASEBOARD_SLIPPY
 	select MAINBOARD_HAS_LPC_TPM
 	select INTEL_INT15
 	select MAINBOARD_HAS_LIBGFXINIT
-	select MAINBOARD_USE_LIBGFXINIT if MAINBOARD_DO_NATIVE_VGA_INIT
 
 if BOARD_GOOGLE_BASEBOARD_SLIPPY