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_handwire/keymap/keymap.c

@@ -0,0 +1,34 @@
+
+#include <zmk/keys.h>
+#include <zmk/keymap.h>
+#include <keymap.h>
+
+bool zmk_handle_key_user(struct zmk_key_event *key_event)
+{
+	switch (key_event->key)
+	{
+	case CC_LOWR:
+		if (key_event->pressed)
+		{
+			zmk_keymap_layer_activate(1);
+		}
+		else
+		{
+			zmk_keymap_layer_deactivate(1);
+		}
+
+		return false;
+	case CC_RAIS:
+		if (key_event->pressed)
+		{
+			zmk_keymap_layer_activate(2);
+		}
+		else
+		{
+			zmk_keymap_layer_deactivate(2);
+		}
+		return false;
+	}
+
+	return true;
+};