Fixes building vb2lib for postcar. Since postcar is an x86ism, add the Kconfig options only for x86. Change-Id: Ib92436bc7270c24689dcf01a47f0c6fe7661814b Signed-off-by: Patrick Georgi <pgeorgi@google.com> Reviewed-on: https://review.coreboot.org/29395 Reviewed-by: Julius Werner <jwerner@chromium.org> Reviewed-by: Werner Zeh <werner.zeh@siemens.com> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>