mb/*/Kconfig: Factor out MAINBOARD_VENDOR

Only some mainboard vendors have a prompt for this option. Let's be fair
and give this ability to everyone.

Change-Id: I03eec7c13d18b42e3c56fb1a43dc665d5dbd1145
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39179
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/mainboard/adlink/Kconfig

@@ -4,7 +4,6 @@ comment "see under vendor LiPPERT"
 # any further boards will then be ADLINK
 
 config MAINBOARD_VENDOR
-	string
 	default "ADLINK"
 
 endif # VENDOR_ADLINK