refactor(split): cmake split clean up.

* Cleaner cmake setup for split build.

app/CMakeLists.txt

@@ -79,17 +79,9 @@ target_sources_ifdef(CONFIG_ZMK_BACKLIGHT app PRIVATE src/behaviors/behavior_bac
 target_sources_ifdef(CONFIG_ZMK_BLE app PRIVATE src/events/battery_state_changed.c)
 target_sources_ifdef(CONFIG_ZMK_BLE app PRIVATE src/battery.c)
 
-if (CONFIG_ZMK_SPLIT_BLE)
-  if (NOT CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
-    target_sources(app PRIVATE src/split_listener.c)
-    target_sources(app PRIVATE src/split/bluetooth/service.c)
-    target_sources(app PRIVATE src/split/bluetooth/peripheral.c)
-    target_sources(app PRIVATE src/events/split_peripheral_status_changed.c)
-  endif()
-  if (CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
-    target_sources(app PRIVATE src/split/bluetooth/central.c)
-  endif()
-endif()
+target_sources_ifdef(CONFIG_ZMK_SPLIT app PRIVATE src/events/split_peripheral_status_changed.c)
+add_subdirectory(src/split)
+
 target_sources_ifdef(CONFIG_USB_DEVICE_STACK app PRIVATE src/usb.c)
 target_sources_ifdef(CONFIG_ZMK_USB app PRIVATE src/usb_hid.c)
 target_sources_ifdef(CONFIG_ZMK_RGB_UNDERGLOW app PRIVATE src/rgb_underglow.c)

app/src/split/CMakeLists.txt

@@ -0,0 +1,6 @@
+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if (CONFIG_ZMK_SPLIT_BLE)
+    add_subdirectory(bluetooth)
+endif()

app/src/split/bluetooth/CMakeLists.txt

@@ -0,0 +1,11 @@
+# Copyright (c) 2022 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+if (NOT CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
+  target_sources(app PRIVATE split_listener.c)
+  target_sources(app PRIVATE service.c)
+  target_sources(app PRIVATE peripheral.c)
+endif()
+if (CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
+  target_sources(app PRIVATE central.c)
+endif()