payloads/U-Boot: Enable the frame buffer

U-Boot normally runs with a display if available. Enable this option so
that the display shows if U-Boot supports it.

Use 'select' rather than 'imply' since the CI complains.

Signed-off-by: Simon Glass <sjg@chromium.org>
Change-Id: Ie5684c6ead30076689f43034675ff9f3531970fe
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59605
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

payloads/external/U-Boot/Kconfig

@@ -3,6 +3,7 @@ if PAYLOAD_UBOOT
 config PAYLOAD_SPECIFIC_OPTIONS
 	def_bool y
 	select PAYLOAD_IS_FLAT_BINARY
+	select WANT_LINEAR_FRAMEBUFFER
 
 choice
 	prompt "U-Boot version"