Add a script to workaround session-migration mtime issue.
requested to merge personal/peat-psuwit/focal_-_workaround-session-migration-mtime into ubports/focal
Long story short, on some devices (e.g. Qualcomm ones) the session- migration script directory's mtime gets wrong, which cause session- migration itself to not consider the directory at all.
Since our rootfs is read-only, the workaround is written so that we can change the directory's mtime without making the rootfs read-write.
Fixes: ubports/development/core/focal-projectmanagement-missing-packages#81 (closed)