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/splash_source.c

@@ -414,7 +414,7 @@ int splash_source_load(struct splash_location *locations, uint size)
 	if (env_splashimage_value == NULL)
 		return -ENOENT;
 
-	bmp_load_addr = simple_strtoul(env_splashimage_value, 0, 16);
+	bmp_load_addr = hextoul(env_splashimage_value, 0);
 	if (bmp_load_addr == 0) {
 		printf("Error: bad splashimage address specified\n");
 		return -EFAULT;