Add rivest cipher 4 (rc4) implementation

Add an implementation of RC4. This will be used by Rockchip booting but may
be useful in other situations.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/rc4.h

@@ -0,0 +1,21 @@
+/*
+ * (C) Copyright 2015 Google, Inc
+ *
+ * (C) Copyright 2008-2014 Rockchip Electronics
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef __RC4_H
+#define __RC4_H
+
+/**
+ * rc4_encode() - encode a buf with the RC4 cipher
+ *
+ * @buf:	Buffer to encode (it is overwrite in the process
+ * @len:	Length of buffer in bytes
+ * @key:	16-byte key to use
+ */
+void rc4_encode(unsigned char *buf, unsigned int len, unsigned char key[16]);
+
+#endif