add -fvisibility=hidden to CC flags, mark XXX_main functions

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

networking/nameif.c

@@ -55,7 +55,7 @@ static struct ether_addr *cc_macaddr(const char *str_macaddr)
 	return mac;
 }
 
-int nameif_main(int argc, char **argv);
+int nameif_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int nameif_main(int argc, char **argv)
 {
 	mactable_t *clist = NULL;