cmake: make main a component again

examples/bluetooth/a2dp_source/CMakeLists.txt

@@ -2,10 +2,5 @@
 # CMakeLists in this exact order for cmake to work correctly
 cmake_minimum_required(VERSION 3.5)
 
-set(MAIN_SRCS
-    main/bt_app_core.c
-    main/main.c
-    )
-
 include($ENV{IDF_PATH}/tools/cmake/project.cmake)
 project(a2dp_source)

examples/bluetooth/a2dp_source/main/CMakeLists.txt

@@ -0,0 +1,5 @@
+set(COMPONENT_SRCS "bt_app_core.c"
+                   "main.c")
+set(COMPONENT_ADD_INCLUDEDIRS ".")
+
+register_component()