save_y2logs should save "journalctl -b"

[ggardet]

save_y2logs should save journalctl -b, not only journalctl --dmesg:
https://github.com/yast/yast-yast2/blob/master/scripts/save_y2logs#L83

It would help when systemd service fails (e.g. rmt)

[ancorgs]

I created a corresponding card to track this in the private trello board used by the YaST Team at SUSE to coordinate and prioritize: https://trello.com/c/uqSvbtGh/4599-improvements-in-savey2logs