Enable alternative implementation of the in-place upgrade process that boots into a squashfs read-only target RHEL filesystem. Intuitively, this can be seen as booting into a target RHEL Live CD.

The primary benefit of this implementation is the possibility to run leapp later into the boot process (after multi-user.target), without fearing the possibility of executing user services that can negatively interfere with the upgrade process.

The feature is implemented as an opt-in unsupported alternative to the standard upgrade mechanism. Enabling the feature affects the way the upgrade initramfs is built and the parameters of the boot entry. The feature will require also additional disk-space, the exact amount is undetermined as the moment.

Implementation: https://github.com/oamg/leapp-repository/pull/1248