layer.conf: adding xxd as a host tool
layer.conf: adding xxd as a host tool

Adding xxd as a host tool as it's a dependent tool when uboot is
configured to use a default environment file

Signed-off-by: Jaewon Lee <1b369288d13b7b5cc9a37dd614e864b90fd18321@xilinx.com>

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