[meta-xilinx] Add support for building BOOT.BIN

[Philip Balister]

I've taken a first pass at adding Mike's work for building BOOT.BIN to
meta-xilinx. I've built BOOT.BIN for the zedboard and booted it. (First
time I booted the zedboard :)

I need to do a little more work to figure out why +'s in the filename
broke the build.

It should still build for machines that do not provide a fsbl.elf binary
and on build machines that do not have bootgen in the path. I need to do
some more checking.

Finally, I need to make sure I can use a bbappend to customize the
BOOT.BIN for an BSP for my custom hardware.

I would like feedback and encourage people to give this a try to see if
we can test more cases :) I really do not want to break builds for
people who will not use this to build BOOT.BIN

https://github.com/balister/meta-xilinx/commit/5c7356c7800f7a29f5b6e47fc89c5139171659e9

Philip