security/vboot: Add selection for firmware slots used by VBOOT

Make the firmware slot configuration in VBOOT selectable. The following
three modes are available:
  -RO only
  -RO + RW_A
  -RO + RW_A + RW_B

The mode "RO only" is the lowest mode with no safety during update.
You can select either RW_A or RW_AB via Kconfig which will add the
selected parttions to the final image.

Change-Id: I278fc060522b13048b00090b8e5261c14496f56e
Signed-off-by: Philipp Deppenwiese <zaolin@das-labor.org>
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/27714
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

src/vendorcode/google/chromeos/Kconfig

@@ -25,6 +25,7 @@ config CHROMEOS
 	select COLLECT_TIMESTAMPS
 	select VBOOT
 	select VPD
+	select VBOOT_SLOTS_RW_AB
 	help
 	  Enable ChromeOS specific features like the GPIO sub table in
 	  the coreboot table. NOTE: Enabling this option on an unsupported