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/bootm.c b/cmd/bootm.c
index 81c6b93..92468d0 100644
--- a/cmd/bootm.c
+++ b/cmd/bootm.c
@@ -112,7 +112,7 @@
 	if (argc > 0) {
 		char *endp;
 
-		simple_strtoul(argv[0], &endp, 16);
+		hextoul(argv[0], &endp);
 		/* endp pointing to NULL means that argv[0] was just a
 		 * valid number, pass it along to the normal bootm processing
 		 *
@@ -240,7 +240,7 @@
 	}
 
 	for (arg = 1; arg < argc; ++arg) {
-		addr = simple_strtoul(argv[arg], NULL, 16);
+		addr = hextoul(argv[arg], NULL);
 		if (image_info(addr) != 0)
 			rcode = 1;
 	}