make define2mk.sed work on FreeBSD

In the thread "[1.3.2-rc1] MPC8548CDS/MPC8555CDS configs fails to link",
the define2mk.sed script was identified as the source of the link
failure on FreeBSD. The problem is that sed(1) does not always support
the '+' operator. It isn't on FreeBSD. The attach patch implements the
equivalent, using the '*' operator instead and should work everywhere.

Signed-off-by: Marcel Moolenaar <marcelm@juniper.net>