mb/*: Specify type of MAINBOARD_DIR once

Specify the type of the `MAINBOARD_DIR` Kconfig symbol once instead of
doing so on each and every mainboard.

Change-Id: If1cc538b0c4938dac193699897b690e402b3c1e8
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56553
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/emulation/qemu-i440fx/Kconfig

@@ -52,7 +52,6 @@ config VBOOT_VBNV_OFFSET
 	default 0x2c
 
 config MAINBOARD_DIR
-	string
 	default "emulation/qemu-i440fx"
 
 config MAINBOARD_PART_NUMBER