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>

common/bedbug.c

@@ -1005,7 +1005,7 @@ long read_number (char *txt)
 	}
 
 	if (txt[0] == '0' && (txt[1] == 'x' || txt[1] == 'X'))	/* hex */
-		val = simple_strtoul (&txt[2], NULL, 16);
+		val = hextoul(&txt[2], NULL);
 	else						/* decimal */
 		val = simple_strtoul (txt, NULL, 10);