ssh service

- dropbear
- generate host keys on first use
- mount /dev/pts

It's not ideal having the host key disappear when the device is
reboot, but without persistent storage the alternative is generating
it at build time. Deferring this problem to another time

modules/s6/scripts/rc.init

@@ -17,6 +17,9 @@ shift
 
 mount -t proc none /proc
 mount -t sysfs none /sys
+# s6-linux-init mounts /dev before this script is called
+mkdir /dev/pts
+mount -t devpts none /dev/pts
 
 mkdir -m 0750 /run/service-state
 chgrp system /run/service-state