fix: resolve split peripheral linker errors on non-split boards

- rgb_underglow_layer.c: fix #if guards to use CONFIG_ZMK_SPLIT check,
  non-split boards now use zmk_keymap_highest_layer_active() directly
- peripheral_layers_nonsplit.c: add stub implementations of split
  peripheral layer functions for non-split builds (linker needs symbols
  even though code paths are never reached at runtime)

Author: alt-0191

app/src/peripheral_layers_nonsplit.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+/*
+ * Stub implementation of peripheral_layers for non-split keyboards.
+ * The split_peripheral_layer_changed event is never raised on non-split boards,
+ * so these functions are never called at runtime, but the linker needs the symbols.
+ */
+
+#include <stdint.h>
+#include <stdbool.h>
+
+void set_peripheral_layers_state(uint32_t new_layers) {}
+
+bool peripheral_layer_active(uint8_t layer) { return false; }
+
+uint8_t peripheral_highest_layer_active(void) { return 0; }

app/src/rgb_underglow_layer.c

@@ -15,7 +15,7 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <zmk/keymap.h>
 #include <zmk/rgb_underglow_layer.h>
 
-#if !IS_ENABLED(CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
+#if IS_ENABLED(CONFIG_ZMK_SPLIT) && !IS_ENABLED(CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
 #include <zmk/split/peripheral_layers.h>
 #endif
 
@@ -75,10 +75,10 @@ uint8_t rgb_underglow_top_layer_with_state(uint32_t state_to_test) {
 }
 
 uint8_t rgb_underglow_top_layer(void) {
-#if IS_ENABLED(CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
-    return zmk_keymap_highest_layer_active();
-#else
+#if IS_ENABLED(CONFIG_ZMK_SPLIT) && !IS_ENABLED(CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
     return peripheral_highest_layer_active();
+#else
+    return zmk_keymap_highest_layer_active();
 #endif
 }
-#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */
+#endif /* DT_HAS_COMPAT_STATUS_OKAY(DT_DRV_COMPAT) */