kern.post.mk: fix PORTSDIR handling

Using subshell's PORTSDIR variable (via $${PORTSDIR}}) seems to be
only working if PORTSDIR is specified directly on the make command
line.

Use ${PORTDIR} here instead so that setting the variable in
/etc/{make,src,src-env}.conf would work (also works for variable
being set on command line or in the environment).

PR: 268299
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D37868