mainboard/google/reef: provide baseboard and variant concepts

To further the ability of multiple variant boards to share code
provide a place to land the split up changes. This patch provides
the tooling using a new Kconfig value, VARIANT_DIR, as well as
the Make plumbing. The directory layout with a single variant,
reef (which is also the baseboard), looks like this:

variants/baseboard - code
variants/baseboard/include/baseboard - headers
variants/reef - code
variants/reef/include/variant - headers

New boards would then add themselves under their board name
within the 'variants' directory.

No split has been done with providing different logic yet.
This is purely a organizational change.

BUG=chrome-os-partner:56677

Change-Id: Ib73a3c8a3729546257623171ef6d8fa7a9f16514
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16418
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

src/mainboard/google/reef/mainboard.c

@@ -21,8 +21,8 @@
 #include <soc/gpio.h>
 #include <soc/nhlt.h>
 #include <vendorcode/google/chromeos/chromeos.h>
-#include "ec.h"
-#include "gpio.h"
+#include <variant/ec.h>
+#include <variant/gpio.h>
 
 static void mainboard_init(void *chip_info)
 {