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.

system/nsh/nsh_main.c

@@ -258,11 +258,7 @@ static int nsh_task(void)
  * Name: nsh_main
  ****************************************************************************/
 
-#ifdef BUILD_MODULE
 int main(int argc, FAR char *argv[])
-#else
-int nsh_main(int argc, char *argv[])
-#endif
 {
   struct sched_param param;