Application.mk and main.c files: Change builtin's entry point from main to xxx_main by macro expansion. This change make the entry point fully compliant with POSIX/ANSI standard.

examples/i2sloop/i2sloop_main.c

@@ -73,11 +73,7 @@ static void _signal_handler(int number)
  * Name: i2sloop_main
  ****************************************************************************/
 
-#ifdef BUILD_MODULE
 int main(int argc, FAR char *argv[])
-#else
-int i2sloop_main(int argc, char *argv[])
-#endif
 {
   FAR struct ap_buffer_s *apb;
   struct audio_buf_desc_s desc;