Set the default hostname using the non-vendor part of board-name and the
"label" / "base" MAC address.

For example with a board_name (1st string in /compatible in dtb) of
foo,model-1234 and MAC address de:ad:be:ef:12:34 will get a hostname of
model-1234-deadbeef1234.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>