plnx-deploy.bbclass: Change hard setting of linux-xlnx to an append
plnx-deploy.bbclass: Change hard setting of linux-xlnx to an append

Issue: CR-1059795

If xen is enabled, linux-xlnx may already have set some of the items for
the plnx-deploy class to deploy.  So only append.

Also in case of an error, instead of crashing, display a warning to the
user that something may be incorrect.

Signed-off-by: Mark Hatle <41765ab18fb75e64ff0efe27f2ebcdb2d6f7e44e@xilinx.com>

Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux,Xilinx/meta-petalinux