Since systemd version 215, adding systemd.debug-shell to the kernel command
line activates the debug shell on tty9 alongside the normal boot. I like the
idea of that, and I'd like to have it in my standard 'rescue' entry in my grub
menu.
Unfortunately, by default update-grub does not allow to customize the rescue
menu entry options. I have just filed #766530
hoping for that to change.
After testing the patch I proposed for /etc/grub.d/10_linux, I now have this
in my /etc/default/grub, with some satisfaction:
GRUB_CMDLINE_LINUX_RECOVERY="systemd.log_target=kmsg systemd.log_level=debug systemd.debug-shell"
Further information:
Thanks to sjoerd and uau on #debian-systemd for their help.