cmake: Add CMakeLists.txt files for all examples

Generating using convert_to_cmake.py, with some minor cleanup

examples/bluetooth/a2dp_source/CMakeLists.txt

@@ -0,0 +1,11 @@
+# The following lines of boilerplate have to be in your project's
+# 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)