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

EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

console-tools/setconsole.c

@@ -18,7 +18,7 @@ static const char setconsole_longopts[] ALIGN1 =
 
 #define OPT_SETCONS_RESET 1
 
-int setconsole_main(int argc, char **argv);
+int setconsole_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int setconsole_main(int argc, char **argv)
 {
 	unsigned long flags;