sshd expects there to be space in /persist/secrets that it can
use to write host keys, but when we make ext4fs images we don't
put any free space in them
It will probably work fine for USB-stick boot (except in the case
where there is > 1 usb device plugged in, so maybe don't do that)
It doesn't matter for TFTP boot because boot.scr overrides the root=
param anyway