Five plugins (soon to be 7) are linking to the DFU plugin just for this simple
segment-aware chunking functionality. Move this into common code to make
building simpler.
Notably, bootloaders for this class of device export an incorrect DFU interface.
Additionally, allow setting the buffer size for the UPLOAD to a larger size
than the defined device transfer size, which allows us to return the full
packet from the larger XMEGA devices.
