Add RPCBuffer.c to construct RPC-related buffers

 - Add `RPCBuffer.c` to construct RPC-related global buffers.
 - Add `RPCBuffer.ht` as [Jinja2][1] template for generating
   `RPCBuffer.h` header.
  - Header defines size of:
   - Serial interface packet.
   - i2c interface packet.
   - Command processor array buffer, which is used for decoding arrays
     received as RPC arguments.

Header generation
=================

Generating default header
-------------------------

A `RPCBuffer.h` header can be generated by using the following command:

    paver generate_rpc_buffer_header

The `generate_rpc_buffer_header` paver task is also a dependency of the
`build_firmware` task, so the header will automatically be generated
when running:

    paver build_firmware

Overriding default header
-------------------------

The header will only be generated if there is no existing file.  This
permits users to make modifications to the header.
Use AVR ifdefs to specialize board buffer settings

Use AVR `#ifdef`s to specialize buffer settings on a per-board basis.
Merge branch 'custom-buffers'
Update README.md