[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/forcedeth.c

@@ -452,7 +452,7 @@ static int reg_delay(int offset, u32 mask,
 		delaymax -= delay;
 		if (delaymax < 0) {
 			if (msg)
-				printf(msg);
+				printf("%s", msg);
 			return 1;
 		}
 	} while ((readl(base + offset) & mask) != target);