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>

src/arch/x86/Kconfig

@@ -162,7 +162,7 @@ config HAVE_CMOS_DEFAULT
 
 config CMOS_DEFAULT_FILE
 	string
-	default "src/mainboard/$(MAINBOARDDIR)/cmos.default"
+	default "src/mainboard/\$(MAINBOARDDIR)/cmos.default"
 	depends on HAVE_CMOS_DEFAULT
 
 config IOAPIC_INTERRUPTS_ON_FSB