Support for JTAG-PDI based Atmel parts

[dragonmux]

This PR establishes support for several things related to the Atmel 8-bit AVR lineup:

• It establishes support for handling AVR parts separately from the rest.
• It establishes support for the JTAG-PDI protocol in BMPF.
• It establishes target support for the ATXMega256A3U processor based atop of this JTAG-PDI support.
• It establishes debug support for that same processor based on our documentation work for the PDI protocol w/ a focus on how debug works

It should be noted that the AVRs are pretty sensitive to how scan is completed which is why #978 had to land first, and the PDI controller is very sensitive to how data is clocked into the target. What the PDI controller does not care about is how fast we do so as JTAG on these parts is decoupled from CPU clock speed completely. You can run the PDI side of things at 10MHz on a processor running at 2 and nothing will hurt for that, except you getting a lot of PDI Delay Byte responses (which we make transparent to the target code for simplicity's sake).