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/yaffs2.c b/cmd/yaffs2.c
index f29ebcc..27fbd1b 100644
--- a/cmd/yaffs2.c
+++ b/cmd/yaffs2.c
@@ -165,8 +165,8 @@
 	}
 
 	filename = argv[1];
-	value = simple_strtoul(argv[2], NULL, 16);
-	numValues = simple_strtoul(argv[3], NULL, 16);
+	value = hextoul(argv[2], NULL);
+	numValues = hextoul(argv[3], NULL);
 
 	printf("Writing value (%lx) %lx times to %s... ", value, numValues,
 	       filename);
@@ -188,7 +188,7 @@
 	}
 
 	filename = argv[1];
-	addr = simple_strtoul(argv[2], NULL, 16);
+	addr = hextoul(argv[2], NULL);
 
 	cmd_yaffs_mread_file(filename, (char *)addr);
 
@@ -207,8 +207,8 @@
 	}
 
 	filename = argv[1];
-	addr = simple_strtoul(argv[2], NULL, 16);
-	size = simple_strtoul(argv[3], NULL, 16);
+	addr = hextoul(argv[2], NULL);
+	size = hextoul(argv[3], NULL);
 
 	cmd_yaffs_mwrite_file(filename, (char *)addr, size);