Kernel panic when set "ssh_key" boot parameter.
Per my understanding, ssh_key takes two form. One is a URI leading to a
downloading during boot up. The other is directly write to
authorized_keys.
But actually, when I take the second way, having my id_rsa.pub content
as the value (403 chars), kernel panic.
(from redmine: issue id 9504, created on 2018-10-03)