[meta-xilinx] Fixes for minized linux-firmware bbappend

[Philip Balister]

I had to make a copy of the bbappend for linux-firmware for
meta-xilinx-contrib for minized. (I'm using another baord that uses the
wifi part) Along the way I found some typos that need fixing. I did a
quick fixup on github and created a pull request. Against master, not
warrior. The branching scheme is hard to follow :)

https://github.com/Xilinx/meta-xilinx/pull/16

Philip