linux-xilinx-machines.inc: Do not clobber COMPATIBLE_MACHINE
linux-xilinx-machines.inc: Do not clobber COMPATIBLE_MACHINE

Do not clobber the value of COMPATIBLE_MACHINE, instead set it via ?=.
This was clobbering the COMPATIBLE_MACHINE values from linux-yocto
causing issues when the meta-xilinx layer was added but not building a
meta-xilinx machine.

Signed-off-by: Nathan Rossi <2e8aa918660411855c6d44d5bb2da677aa033255@nathanrossi.com>

Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx