Kconfig: comply to Linux 5.3's Kconfig language rules

Kconfig became stricter on what it accepts, so accomodate before
updating to a new release.

Change-Id: I92a9e9bf0d557a7532ba533cd7776c48f2488f91
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37156
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/emulation/qemu-i440fx/Kconfig

@@ -42,7 +42,7 @@ config VBOOT_VBNV_OFFSET
 
 config MAINBOARD_DIR
 	string
-	default emulation/qemu-i440fx
+	default "emulation/qemu-i440fx"
 
 config MAINBOARD_PART_NUMBER
 	string