Package: src:linux Version: 4.8.15-2 $ cat /proc/cmdline BOOT_IMAGE=/boot/vmlinuz-4.8.0-2-amd64 root=UUID=50c0e259-55bb-4fc0-867e-63160edef01f ro single systemd.log_level=debug systemd.log_target=kmsg log_buf_len=262144 enforcing=0 panic=33 earlyprintk=vga worked fine. But if boot_delay=500 is added to it, all that happens is two lines are printed on the screen, and one has to power cycle.