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/cbfs.c b/cmd/cbfs.c
index 10c2c92..8a61f2c 100644
--- a/cmd/cbfs.c
+++ b/cmd/cbfs.c
@@ -22,7 +22,7 @@
 		return 0;
 	}
 	if (argc == 2) {
-		end_of_rom = simple_strtoul(argv[1], &ep, 16);
+		end_of_rom = hextoul(argv[1], &ep);
 		if (*ep) {
 			puts("\n** Invalid end of ROM **\n");
 			return 1;
@@ -58,9 +58,9 @@
 	}
 
 	/* parse offset and count */
-	offset = simple_strtoul(argv[1], NULL, 16);
+	offset = hextoul(argv[1], NULL);
 	if (argc == 4)
-		count = simple_strtoul(argv[3], NULL, 16);
+		count = hextoul(argv[3], NULL);
 	else
 		count = 0;