Allow keyboard_scan_config to be customized per-board

By default it's not, but if you #define CONFIG_CUSTOM_KEYSCAN, you can add
your own settings in board.c

BUG=chrome-os-partner:18343
TEST=build link, bds
BRANCH=none

Change-Id: I2a9dd48fd7f4610bc39dcc59e59a3fedec539e28
Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48994
Reviewed-by: Randall Spangler <rspangler@chromium.org>

include/keyboard_scan.h

@@ -42,6 +42,10 @@ void keyboard_scan_init(void);
  * Return a pointer to the keyboard scan config.
  */
 struct keyboard_scan_config *keyboard_scan_get_config(void);
+/*
+ * Which is probably this.
+ */
+extern struct keyboard_scan_config keyscan_config;
 
 /* Key held down at keyboard-controlled reset boot time. */
 enum boot_key {