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/biostar/Kconfig

@@ -13,6 +13,7 @@
 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 ## GNU General Public License for more details.
 ##
+
 if VENDOR_BIOSTAR
 
 choice
@@ -26,7 +27,6 @@ source "src/mainboard/biostar/*/Kconfig"
 
 
 config MAINBOARD_VENDOR
-	string
 	default "Biostar"
 
 endif # VENDOR_BIOSTAR