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/unzip.c b/cmd/unzip.c
index 9b28328..3d1f5f3 100644
--- a/cmd/unzip.c
+++ b/cmd/unzip.c
@@ -18,11 +18,11 @@
 
 	switch (argc) {
 		case 4:
-			dst_len = simple_strtoul(argv[3], NULL, 16);
+			dst_len = hextoul(argv[3], NULL);
 			/* fall through */
 		case 3:
-			src = simple_strtoul(argv[1], NULL, 16);
-			dst = simple_strtoul(argv[2], NULL, 16);
+			src = hextoul(argv[1], NULL);
+			dst = hextoul(argv[2], NULL);
 			break;
 		default:
 			return CMD_RET_USAGE;
@@ -60,11 +60,11 @@
 	if (ret < 0)
 		return CMD_RET_FAILURE;
 
-	addr = (unsigned char *)simple_strtoul(argv[3], NULL, 16);
-	length = simple_strtoul(argv[4], NULL, 16);
+	addr = (unsigned char *)hextoul(argv[3], NULL);
+	length = hextoul(argv[4], NULL);
 
 	if (5 < argc) {
-		writebuf = simple_strtoul(argv[5], NULL, 16);
+		writebuf = hextoul(argv[5], NULL);
 		if (6 < argc) {
 			startoffs = simple_strtoull(argv[6], NULL, 16);
 			if (7 < argc)