Testing release for BTRFS/Snapshots feature

[Stefan Schantl <stefan.schantl@ipfire.org>]

Hello list followers,

a lot of time has been passed since the initial support for BTRFS has
been introduced in IPFire. Sadly the further development process has
been slowed down significantly - mainly because of personal reasons and
as a result of them, the lack of spare time on my side.

All the more I'm very happy to announce the next milestone on this list
and share the current development state. The roadmap on our wiki page
can be found here: https://www.ipfire.org/docs/roadmap/btrfs

The code to take snapshots, manage them, boot into a certain snapshot
or to restore them finally has been arrived. All these actions easily
can be accessed from the CGI page for backups on the IPFire WUI. On
this page a new section called "Snapshots" will be displayed in case
BTRFS has been chosen during the install process of IPFire.

The page layout and style is not final yet, but everything is expected
to work properly. In case you have any ideas how to improve the look
and feel of the CGI page please provide some feedback here.

The grub boot menu automatically will be extended when the first
snapshot has been created with an additional entry to access and boot
into a select-able snapshot in case the system broke or for any other
testing purposes.

I've also extend the code of the pakfire package manager to create a
snapshot before installing a core update.

To summarize the above the snapshot feature would be a big step
forward, because it a allows anybody to go back if some unexpected
error occur after an update, a testing update breaks anything, for
hunting when a certain bug has been seen for the first time, etc...

To took the last remaining steps and get the feature into the
distribution some more testing and feedback is required. Therefore I've
backed and uploaded an ISO image for all who are interested and want to
join testing.

Sadly IPFire has to be re-installed with BTRFS because there is no way
to migrate an existing installation to a different file system.

The current (and may later) images can be found here: 

https://people.ipfire.org/~stevee/BTRFS/

According to testing and feedback there are some additional questions
which appeared during development and needs some attention:

* Should BTRFS become the default selected file system during
installation? (At the moment this is ext4)

* Should the pre-generated images use BTRFS? (At the moment they are
also ext4)

* Do we want an automatic snapshot for each single core update, or do
we want a single one in case multiple core updates will be installed in
one go? (Currently I've implemented a snapshot for each single update)

* Maybe I missed something - feel free to ask any questions or share
your opinions here.

A big thanks in advance to anyone who wants to contribute and helps to
finalize this amazing feature.

Best regards,

-Stefan