mb/purism/librem_skl: rename variant directories

Since the same variant dirs are used by multiple versions of the
same board, drop the v2/v3 labels.

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
Change-Id: Id913e31ab52043e49769be9d3ebf6e71ecb0c856
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40912
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner

src/mainboard/purism/librem_skl/Kconfig

@@ -20,8 +20,8 @@ config IRQ_SLOT_COUNT
 
 config VARIANT_DIR
 	string
-	default "librem13v2" if BOARD_PURISM_LIBREM13_V2 || BOARD_PURISM_LIBREM13_V4
-	default "librem15v3" if BOARD_PURISM_LIBREM15_V3 || BOARD_PURISM_LIBREM15_V4
+	default "librem13" if BOARD_PURISM_LIBREM13_V2 || BOARD_PURISM_LIBREM13_V4
+	default "librem15" if BOARD_PURISM_LIBREM15_V3 || BOARD_PURISM_LIBREM15_V4
 
 config MAINBOARD_FAMILY
 	string