zynq: fix detection of the zybo board

The board_name was being tested for "xlnx,zynq-zybo",
but the .dts compatible string is "digilent,zynq-zybo"

Also, sorted the boards alphabetically, and added an
error message for unsupported boards

Signed-off-by: Luis Araneda <luaraneda@gmail.com>