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/strings.c b/cmd/strings.c
index 85fdc34..bf348af 100644
--- a/cmd/strings.c
+++ b/cmd/strings.c
@@ -18,9 +18,9 @@
 		return CMD_RET_USAGE;
 
 	if ((flag & CMD_FLAG_REPEAT) == 0) {
-		start_addr = (char *)simple_strtoul(argv[1], NULL, 16);
+		start_addr = (char *)hextoul(argv[1], NULL);
 		if (argc > 2)
-			last_addr = (char *)simple_strtoul(argv[2], NULL, 16);
+			last_addr = (char *)hextoul(argv[2], NULL);
 		else
 			last_addr = (char *)-1;
 	}