mb/purism/librem_skl: Convert to use override devicetree

Since the variants' devicetrees are almost identical, convert to
using an overridetree setup for simplicity.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Change-Id: I3dac62a649e12ea2498d3ecafe03fd0d62af5f2b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40911
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/src/mainboard/purism/librem_skl/Kconfig b/src/mainboard/purism/librem_skl/Kconfig
index ca1582a..be4c391e 100644
--- a/src/mainboard/purism/librem_skl/Kconfig
+++ b/src/mainboard/purism/librem_skl/Kconfig
@@ -39,9 +39,9 @@
 	string
 	default "purism/librem_skl"
 
-config DEVICETREE
+config OVERRIDE_DEVICETREE
 	string
-	default "variants/$(CONFIG_VARIANT_DIR)/devicetree.cb"
+	default "variants/$(CONFIG_VARIANT_DIR)/overridetree.cb"
 
 config MAX_CPUS
 	int