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/adtimg.c b/cmd/adtimg.c
index aa94300..f4b5cbf 100644
--- a/cmd/adtimg.c
+++ b/cmd/adtimg.c
@@ -27,7 +27,7 @@
 	if (argc != 2)
 		return CMD_RET_USAGE;
 
-	hdr_addr = simple_strtoul(argv[1], &endp, 16);
+	hdr_addr = hextoul(argv[1], &endp);
 	if (*endp != '\0') {
 		printf("Error: Wrong image address '%s'\n", argv[1]);
 		return CMD_RET_FAILURE;