Add Arduino library
Add memory/register defs for Arduino library
arduino: Add readback test
arduino: readback_test tests VRAM
arduino: add register bit for enabling VRAM access
arduino: Fix number of sprite regs

There are 16 regs (32 bytes) per sprite, not 32 regs.
arduino: Basic DuinoCube system shield support
arduino: Add DuinoCube RPC header with status codes
arduino: Add RPC command enums
arduino: Add scroll registers
arduino: implement RPC test functions
arduino: include RPC definitions in DuinoCube header
arduino: Add basic RPC and serial RAM code
arduino: Simplify RPC mechanism / codes / structs

Instead of using separate RPC status codes and commands, combine the two
to make things simpler.  The protocol is also a little bit simpler (fewer
stages).
arduino: Make serial RAM access functions public

Also move resetRPCServer() to match function order in header.