Mount efivarfs when needed (Closes: #1051607)
The file mount.conf is made more similar to the version in the package Calamares.
- efivarfs is added, because Calamares no longer looks for extraMountsEfi. https://sources.debian.org/src/calamares/3.2.61-1/src/modules/mount/main.py/#L236 https://sources.debian.org/src/calamares/3.3.0~alpha6-2/src/modules/mount/main.py/#L348
- /dev/pts is added, because apt complains about it missing. apt can then save the installation log files.