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/board/synopsys/hsdk/env-lib.c b/board/synopsys/hsdk/env-lib.c
index 235f295..e225838 100644
--- a/board/synopsys/hsdk/env-lib.c
+++ b/board/synopsys/hsdk/env-lib.c
@@ -252,7 +252,7 @@
 	char *endp = argv[1];
 
 	if (map[i].type == ENV_HEX)
-		map[i].val->val = simple_strtoul(argv[1], &endp, 16);
+		map[i].val->val = hextoul(argv[1], &endp);
 	else
 		map[i].val->val = simple_strtoul(argv[1], &endp, 10);