uboot-phytec

boards.cfg: change "<none>" in the board field to "-"

In the previous commit, all the board fields were filled.

Now we can use "-" in the board field for a different meaning.

Going forward, "-" stands for no board directory
as in cpu, soc, vendor fields.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>