mainboard/google/kahlee: Prepare for variants

Move files that are particularly specific to the mainboard into the
variant directory.  Files that only have small areas of mainboard
specific pieces use #if to separate between the boards.

Add memory.c to split out the variant board id into a weak function.
Add baseboard/gpio.h to satisfy the build - this will be updated in the
next commit.

BUG=b:68293392

Change-Id: I7c1beb45f571f2547f3b5b0d7ec78923d0cec761
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/22185
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/kahlee/chromeos.c

@@ -19,10 +19,7 @@
 #include <boot/coreboot_tables.h>
 #include <console/console.h>
 #include <gpio.h>
-
-/* SPI Write protect */
-#define CROS_WP_GPIO		GPIO_142
-#define GPIO_EC_IN_RW		GPIO_15
+#include <variant/gpio.h>
 
 void fill_lb_gpios(struct lb_gpios *gpios)
 {