fix: Reduce RAM usage in the keymap

* When not building with runtime keymap support, make the keymap
  const.

app/src/keymap.c

@@ -83,7 +83,7 @@ static uint8_t keymap_layer_orders[ZMK_KEYMAP_LAYERS_LEN];
                     (DT_INST_FOREACH_CHILD_SEP(0, TRANSFORMED_LAYER, (, ))),                       \
                     (DT_INST_FOREACH_CHILD_STATUS_OKAY_SEP(0, TRANSFORMED_LAYER, (, ))))};
 
-KEYMAP_VAR(zmk_keymap, )
+KEYMAP_VAR(zmk_keymap, COND_CODE_1(IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE), (), (const)))
 
 #if IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)
 
@@ -238,8 +238,6 @@ zmk_keymap_get_layer_binding_at_idx(zmk_keymap_layer_id_t layer_id, uint8_t bind
 
 static uint8_t zmk_keymap_layer_pending_changes[ZMK_KEYMAP_LAYERS_LEN][PENDING_ARRAY_SIZE];
 
-#endif // IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)
-
 int zmk_keymap_set_layer_binding_at_idx(zmk_keymap_layer_id_t layer_id, uint8_t binding_idx,
                                         struct zmk_behavior_binding binding) {
     if (binding_idx >= ZMK_KEYMAP_LEN) {
@@ -248,18 +246,24 @@ int zmk_keymap_set_layer_binding_at_idx(zmk_keymap_layer_id_t layer_id, uint8_t
 
     ASSERT_LAYER_VAL(layer_id, -EINVAL)
 
-#if IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)
     uint8_t *pending = zmk_keymap_layer_pending_changes[layer_id];
 
     WRITE_BIT(pending[binding_idx / 8], binding_idx % 8, 1);
-#endif // IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)
 
     // TODO: Need a mutex to protect access to the keymap data?
     memcpy(&zmk_keymap[layer_id][binding_idx], &binding, sizeof(binding));
 
     return 0;
 }
 
+#else
+
+int zmk_keymap_set_layer_binding_at_idx(zmk_keymap_layer_id_t layer_id, uint8_t binding_idx,
+                                        struct zmk_behavior_binding binding) {
+    return -ENOTSUP;
+}
+
+#endif // IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)
 #if IS_ENABLED(CONFIG_ZMK_KEYMAP_LAYER_REORDERING)
 
 #if IS_ENABLED(CONFIG_ZMK_KEYMAP_SETTINGS_STORAGE)