sshd: Move hostkeys to /var/lib/ssh

This means keys are persistent even when /etc is not.

board/common/rootfs/etc/finit.d/available/sshd.conf

@@ -0,0 +1,2 @@
+task [S] /lib/infix/ssh-hostkeys --
+service [2345789] <usr/ssh-hostkeys> /usr/sbin/sshd -D -- OpenSSH daemon

board/common/rootfs/etc/finit.d/enabled/sshd.conf

@@ -0,0 +1 @@
+../available/sshd.conf

board/common/rootfs/etc/ssh/sshd_config

@@ -0,0 +1 @@
+Include /etc/ssh/sshd_config.d/*.conf

board/common/rootfs/etc/ssh/sshd_config.d/var-keys.conf

@@ -0,0 +1,3 @@
+HostKey /var/lib/ssh/ssh_host_rsa_key
+HostKey /var/lib/ssh/ssh_host_ecdsa_key
+HostKey /var/lib/ssh/ssh_host_ed25519_key

board/common/rootfs/lib/infix/ssh-hostkeys

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+for type in ecdsa ed25519 rsa; do
+    [ -f /var/lib/ssh/ssh_host_${type}_key ] \
+	|| ssh-keygen -N "" -t $type -f /var/lib/ssh/ssh_host_${type}_key \
+	|| exit 1
+done
+
+initctl cond set ssh-hostkeys