boards.cfg: Place "-" in the 8th field if it is the same as 7th

The 8th field of boards.cfg takes the form:
     <board config name>[:comma separated config options]

We should describe explicitely the 8th field only when it is necessary
to do so.
Specify "-" in the 8th field if it is the same as 7th field.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>