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/pcap.c b/cmd/pcap.c
index d0172f3..c751980 100644
--- a/cmd/pcap.c
+++ b/cmd/pcap.c
@@ -18,7 +18,7 @@
 	if (argc != 3)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
+	addr = hextoul(argv[1], NULL);
 	size = simple_strtoul(argv[2], NULL, 10);
 
 	return pcap_init(addr, size) ? CMD_RET_FAILURE : CMD_RET_SUCCESS;