This commit adds the requirement for a new kernel command line file at
`/etc/snap-pac-uki/kernel-cmd-current`. When generating current ukis, this file
will be used for the non-fallback variant instead of the generic `kernel-cmd`.
The reason for this change is hibernating. When saving the system state to RAM,
users might not want snapshot varints to resume from there. Previosly, this
would not have been a possible configuration.