Stuff /usr/share/licenses into a squashfs

[iliana]

Issue #, if available: n/a

Description of changes:
Makes the contents of /usr/share/licenses a squashfs on disk that is auto-mounted at boot.

Tested on an aws-k8s AMI, including the changes from #719 — without hardlinking files:

bash-5.0# du -sh /usr/share/licenses/ 
6.9M    /usr/share/licenses/

After hardlinking:

bash-5.0# du -sh /usr/share/licenses/
4.4M    /usr/share/licenses/

Size of the squashfs:

bash-5.0# ls -lh /usr/share/thar/licenses.squashfs 
-rw-r--r-- 1 root root 320K Feb 12 16:47 /usr/share/thar/licenses.squashfs

(Hardlinking doesn't save any space in the squashfs, because there is already duplicate file detection. So it's not included here.)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.