add late_command

preseed.cfg

@@ -427,4 +427,9 @@ d-i debian-installer/exit/poweroff boolean true
 # directly, or use the apt-install and in-target commands to easily install
 # packages and run commands in the target system.
 #d-i preseed/late_command string apt-install zsh; in-target chsh -s /bin/zsh
++# commands to execute after installation finished
 
+# prepare root's .ssh/ directory to later put the SSH keys into
+d-i preseed/late_command string \
+  in-target mkdir /root/.ssh
++