Remove extra boards from LIST_ixp

pdnb3 and scpu are explicitly on LIST_ixp, even though they are
also specified in boards.cfg as having cpu ixp. This means that
they will be built twice when doing ./MAKEALL ixp, or ./MAKEALL arm.

This was pointless before, but actually breaks things if you launch
both builds at the same time, as they overwrite each other.

Signed-off-by: Andy Fleming <afleming@freescale.com>