Introduce rbutil/mkzenboot, a replacement for tools/mkzenboot

The old tools/mkzenboot has a number of problems: very hard to maintain,
poor integration with rbutil. Restart from scratch by recycling all the
crypto and descrambling code, rewrite the actual firmware modification part
to handle all scenarios in a much clearer way. The code is ready to be
integrated into Rockbox Utility, by using the very similar interface to
mkimxboot.
I copied all the keys from the old mkzenboot, so it can potentially support
the older Creative ports, but since this is untested, I prefer not do so
at the moment. However, I did add a "mixed" boot option to support the
dualboot style used in the older ports.

Change-Id: I80cfc48fa78187baa1b1692e8a30ec7137cea37b

rbutil/mkzenboot/md5.h

@@ -0,0 +1,25 @@
+#ifndef _MD5_H
+#define _MD5_H
+
+#ifndef uint8
+#define uint8  unsigned char
+#endif
+
+#ifndef uint32
+#define uint32 unsigned long int
+#endif
+
+typedef struct
+{
+    uint32 total[2];
+    uint32 state[4];
+    uint8 buffer[64];
+}
+md5_context;
+
+void md5_starts( md5_context *ctx );
+void md5_update( md5_context *ctx, uint8 *input, uint32 length );
+void md5_finish( md5_context *ctx, uint8 digest[16] );
+
+#endif /* md5.h */
+