dropbear/openssh: Lower priority of key generation
Where we have images with PAM+systemd, serial login can be extremely
slow. The load generated by key generation does slow down the rest
of the boot process.
Lower the priority level of these systemd services, since we'd
prefer to have the rest of the system boot more effectively.
This doesn't "solve" the slow systemd boot issues but does help.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
