Kconfig: Escape variable to accommodate new Kconfig versions

Kconfig 4.17 started using the $(..) syntax for environment variable
expansion while we want to keep expansion to the build system.
Older Kconfig versions (like ours) simply drop the escapes, not
changing the behavior.

While we could let Kconfig expand some of the variables, that only
splits the handling in two places, making debugging harder and
potentially messing with reproducible builds (e.g. when paths end up
in configs), so escape them all.

Change-Id: Ibc4087fdd76089352bd8dd0edb1351ec79ea4faa
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42481
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>
diff --git a/src/mainboard/purism/librem_skl/Kconfig b/src/mainboard/purism/librem_skl/Kconfig
index c86ebf1..cd8fc66 100644
--- a/src/mainboard/purism/librem_skl/Kconfig
+++ b/src/mainboard/purism/librem_skl/Kconfig
@@ -45,7 +45,7 @@
 
 config OVERRIDE_DEVICETREE
 	string
-	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb"
+	default "variants/\$(CONFIG_VARIANT_DIR)/overridetree.cb"
 
 config MAX_CPUS
 	int
@@ -59,7 +59,7 @@
 # Override the default variant behavior, since same VBT
 # is used for all variants.
 config INTEL_GMA_VBT_FILE
-	default "src/mainboard/$(MAINBOARDDIR)/data.vbt"
+	default "src/mainboard/\$(MAINBOARDDIR)/data.vbt"
 
 config DIMM_MAX
 	int