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>

lib/vsprintf.c

@@ -858,7 +858,7 @@ bool str2long(const char *p, ulong *num)
 {
 	char *endptr;
 
-	*num = simple_strtoul(p, &endptr, 16);
+	*num = hextoul(p, &endptr);
 	return *p != '\0' && *endptr == '\0';
 }