Lock FreeBSD ports after build

Summary:
When a port is built, it will be reinstalled by next run
of pkg update. To prevent this behavior, packages must be
locked.

This is a follow-up for 8468dd24.

Test Plan: Tested on WindRiver

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2137