Description: Rebase boom-boot to 1.6.1.

Upstream will release boom-1.6.1 this week:

https://github.com/snapshotmanager/boom/pull/32

This is primarily a bug fix release and addresses several problems found in 1.6.0:

8bfc684 boom: bump release to 1.6.1
40d9663 boom.bootloader: repair boom entries with mismatched boot_id
9c03c25 boom.bootloader: make BOOM_ENTRIES_PATTERN more strict
7a134cf Revert "boom.cache: ignore foreign boot entries when reference counting"
800ca09 command: fix docstring typo ('or``OsError``')
1ec7faa  doc: update Grub2 docs to add removal of submenu support in 1.4
664e80e  snapshot-remount-fs: use correct systemd unit path
94616c7 snapshot-remount-fs: limit maxsplit when splitting kernel arguments
6158e32  dist: update spec file