Initial refactor to have a shield/board have one default keymap.

* Start of work of focusing on out-of-tree user keymaps.

Add ZMK_CONFIG support.

* General external configuration supported via ZMK_CONFIG,
  which should point to a configuration directory.
* Priority order of overlay files to select.
  * ${SHIELD}_${BOARD}.overlay
  * ${SHIELD}.overlay
  * ${BOARD}.overlay
  * default.overlay
* Equivalent search order for .conf files for initial Kconfig
  values.

app/boards/shields/petejohanson_proton_handwire/keymap/include/keymap.h

@@ -0,0 +1,9 @@
+
+#include <dt-bindings/zmk/keys.h>
+
+#define DEFAULT 0
+#define RAISE 1
+#define LOWER 2
+
+#define CC_RAIS ZC_CSTM(RAISE)
+#define CC_LOWR ZC_CSTM(LOWER)

app/boards/shields/petejohanson_proton_handwire/keymap/keymap.c

@@ -10,22 +10,22 @@ bool zmk_handle_key_user(struct zmk_key_event *key_event)
 	case CC_LOWR:
 		if (key_event->pressed)
 		{
-			zmk_keymap_layer_activate(1);
+			zmk_keymap_layer_activate(LOWER);
 		}
 		else
 		{
-			zmk_keymap_layer_deactivate(1);
+			zmk_keymap_layer_deactivate(LOWER);
 		}
 
 		return false;
 	case CC_RAIS:
 		if (key_event->pressed)
 		{
-			zmk_keymap_layer_activate(2);
+			zmk_keymap_layer_activate(RAISE);
 		}
 		else
 		{
-			zmk_keymap_layer_deactivate(2);
+			zmk_keymap_layer_deactivate(RAISE);
 		}
 		return false;
 	}

app/boards/shields/petejohanson_proton_handwire/keymaps/default/include/keymap.h

@@ -1,5 +0,0 @@
-
-#include <dt-bindings/zmk/keys.h>
-
-#define CC_RAIS ZC_CSTM(1)
-#define CC_LOWR ZC_CSTM(2)