I had a system which failed to boot. The last lines printed to the screen, which were not relevant to the problem, were:

 [19.957783] async_tx: api initialized (async)
 [20.899732] Btrfs loaded, crc32c=crc32c-intel

Then a little bit later:

 [32.469926] random: crng init done
 [32.470551] random: 7 urandom warning(s) missed due to ratelimiting

The full problem description is here. And the solution is here (careful about the typo on that page, the only relevant setting is GRUB_CMDLINE_LINUX_DEFAULT).

Basically the problem was a bogus GRUB_CMDLINE_LINUX_DEFAULT setting in /etc/default/grub applied by my salt config. I fixed the setting which fixed the issue.