global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/source.c

@@ -179,7 +179,7 @@ static int do_source(struct cmd_tbl *cmdtp, int flag, int argc,
 		      fit_uname, addr);
 #endif
 	} else {
-		addr = simple_strtoul(argv[1], NULL, 16);
+		addr = hextoul(argv[1], NULL);
 		debug("*  source: cmdline image address = 0x%08lx\n", addr);
 	}