[meta-xilinx] Add support for building a working SPL and add support for the zc702.

[Philip Balister]

I put together an example of how to build a working boot.bin for the
zc702 board. I used the GPL versions of ps7_init.[ch] to avoid license
incompatibility. You should be able to use a bbappend file to customize
for your specific  board.

I also check for a uEnv.txt file and an fpga.bin file and install it in
the image directory if they are present.

Philip