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/nvedit.c b/cmd/nvedit.c
index d14ba10..02a99b4 100644
--- a/cmd/nvedit.c
+++ b/cmd/nvedit.c
@@ -358,7 +358,7 @@
 
 	s = env_get(varname);
 	if (s)
-		value = simple_strtoul(s, &endp, 16);
+		value = hextoul(s, &endp);
 	if (!s || endp == s)
 		return default_val;
 
@@ -984,7 +984,7 @@
 			case 's':		/* size given */
 				if (--argc <= 0)
 					return cmd_usage(cmdtp);
-				size = simple_strtoul(*++argv, NULL, 16);
+				size = hextoul(*++argv, NULL);
 				goto NXTARG;
 			case 't':		/* text format */
 				if (fmt++)
@@ -1001,7 +1001,7 @@
 	if (argc < 1)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[0], NULL, 16);
+	addr = hextoul(argv[0], NULL);
 	ptr = map_sysmem(addr, size);
 
 	if (size)
@@ -1140,11 +1140,11 @@
 	if (sep != '\n' && crlf_is_lf )
 		crlf_is_lf = 0;
 
-	addr = simple_strtoul(argv[0], NULL, 16);
+	addr = hextoul(argv[0], NULL);
 	ptr = map_sysmem(addr, 0);
 
 	if (argc >= 2 && strcmp(argv[1], "-")) {
-		size = simple_strtoul(argv[1], NULL, 16);
+		size = hextoul(argv[1], NULL);
 	} else if (chk) {
 		puts("## Error: external checksum format must pass size\n");
 		return CMD_RET_FAILURE;