Added support for my AVR128DA28 board.

[TheEccentricGenius]

As per your request I have created a issue for the addition of my board to the supported boards in the core.

I have created and tested the variants file extensively and all appears to be functioning correctly, I have also compiled the hexs for the optiboot bootloader due to the fact I moved the builtin LED to a different pin, I have added it to the boards.txt under the words Produino or Uno 4u.

I have included all the above files on my GitHub for temporary access to anyone else who has the board, here is the GitHub files link.

I will be responsible for any issues that the variants file may create, as I have stated clearly in the opening comments of the variants file.

I have tried to retain all your comments, the only comments that were removed was about it supporting the DB series which this particular board will not be doing. To be clear, I am not currently planning on adding support for anything except the AVR128DA28 chip.

I have removed the option for the reset pin being used as a input and the clock only has internal source options, I have also removed the option of being able to reprogram the board via a alternate serial port. I removed the 48 pin I2C option too. I have removed all options that were related to parts with pin counts over 28 pins, which included millis/micros timers and SPM options.

And to be sure there is no confusion, I added my board choices to the already existing supported chips, I have touched nothing that you have done only added. My boards in the boards.txt file should be the last boards, and are clearly marked with the words UNO 4u in the heading.

Everything else is as you originally designed, implemented and intended for it to be.