Source functions from stackrc

stackrc now requires GITREPO, GITBRANCH, GITDIR and has been
dependent on functions for a while (is_package_installed).  Ensure
we source the required functions file when stackrc is loaded.  Avoids
unexpected issues in grenade where they may or may not have been loaded
depending on the configuration.

Closes-bug: #1377274

Change-Id: I5027cfad07af0de7ff39f424601d6f7ec5dcadae
(cherry picked from commit 0e4cd038287bcf36ff31c4e7b22266051198b44c)