[makefile] Add -Wformat-nonliteral as an extra warning category

-Wformat-nonliteral is not enabled by -Wall and needs to be explicitly
 specified.

Modified the few files that use nonliteral format strings to work with
this new setting in place.

Inspired by a patch from Carl Karsten <carl@personnelware.com> and an
identical patch from Rorschach <r0rschach@lavabit.com>.

src/drivers/net/3c595.c

@@ -363,7 +363,7 @@ vxgetlink(void)
         if (n > 0) {
           printf("/");
 	}
-        printf(conn_tab[k].name);
+        printf("%s", conn_tab[k].name );
         n++;
       }
     }