mb/*/*/Kconfig: guard board name in quotes

New kconfig dislikes unquoted slashes.

Change-Id: Ief242de081071021b9c904a24535d025f6674270
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42480
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/protectli/vault_bsw/Kconfig b/src/mainboard/protectli/vault_bsw/Kconfig
index 499bad4..5850995 100644
--- a/src/mainboard/protectli/vault_bsw/Kconfig
+++ b/src/mainboard/protectli/vault_bsw/Kconfig
@@ -15,7 +15,7 @@
 
 config MAINBOARD_DIR
 	string
-	default protectli/vault_bsw
+	default "protectli/vault_bsw"
 
 config VARIANT_DIR
 	string