[meta-xilinx] Some fixes for meta-xilinx

[Philip Balister]

I've got some fixes here in case people are having trouble with
meta-xilinx and master. I'd like to see the patch with the machine
overrides go into meta-xilinx, without them, there are clashed with
other bsp's using linux-yocto_3.8.

The device generation fix is wrong, but it does let the zedbaord kernel
build succeed.

https://github.com/balister/meta-xilinx/commits/philip-work

Philip