Kconfig: Escape variables

New kconfig parsers interpret $(var) themselves, leading to empty
fields. Old kconfig understands \$(var), so use that.

Change-Id: I927fc9dc7a66211bfe51d4324cf7c51b555ea3a8
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55912
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/system76/whl-u/Kconfig b/src/mainboard/system76/whl-u/Kconfig
index e93b421..a734547 100644
--- a/src/mainboard/system76/whl-u/Kconfig
+++ b/src/mainboard/system76/whl-u/Kconfig
@@ -32,7 +32,7 @@
 
 config OVERRIDE_DEVICETREE
 	string
-	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb"
+	default "variants/\$(CONFIG_VARIANT_DIR)/overridetree.cb"
 
 config MAINBOARD_PART_NUMBER
 	string