try setting  cpu='$cpu' in termrc, with the cpu=... entry in ndb.

the auth=, cpu= ... names in ndb are typically used when cs (via dial) is asked to
look for $X for some X as part of a dial string (eg, net!$auth!fsauth).
since the cpu command puts the value of the environment variable cpu
in a dial string, setting it to the literal '$cpu' puts that in the dial string,
where cs will translate it by looking in ndb.

see the description of metanames in ndb(6)