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>
diff --git a/cmd/osd.c b/cmd/osd.c
index 703d640..8557894 100644
--- a/cmd/osd.c
+++ b/cmd/osd.c
@@ -99,10 +99,10 @@
 		return CMD_RET_FAILURE;
 	}
 
-	x = simple_strtoul(argv[1], NULL, 16);
-	y = simple_strtoul(argv[2], NULL, 16);
+	x = hextoul(argv[1], NULL);
+	y = hextoul(argv[2], NULL);
 	hexstr = argv[3];
-	count = (argc > 4) ? simple_strtoul(argv[4], NULL, 16) : 1;
+	count = (argc > 4) ? hextoul(argv[4], NULL) : 1;
 
 	buflen = strlen(hexstr) / 2;
 
@@ -148,9 +148,9 @@
 		return CMD_RET_FAILURE;
 	}
 
-	x = simple_strtoul(argv[1], NULL, 16);
-	y = simple_strtoul(argv[2], NULL, 16);
-	color = simple_strtoul(argv[3], NULL, 16);
+	x = hextoul(argv[1], NULL);
+	y = hextoul(argv[2], NULL);
+	color = hextoul(argv[3], NULL);
 	text = argv[4];
 
 	res = video_osd_print(osd_cur, x, y, color, text);
@@ -176,8 +176,8 @@
 		return CMD_RET_FAILURE;
 	}
 
-	x = simple_strtoul(argv[1], NULL, 16);
-	y = simple_strtoul(argv[2], NULL, 16);
+	x = hextoul(argv[1], NULL);
+	y = hextoul(argv[2], NULL);
 
 	res = video_osd_set_size(osd_cur, x, y);
 	if (res) {