Use an override for for sshd instead of overwriting the service file

Don't overwrite upstream's .service file, and instead use an override
file for making sure the host key(s) are generated.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

Author: saiarcot895

build_debian.sh

@@ -407,7 +407,8 @@ sudo sed -i 's/LOAD_KEXEC=true/LOAD_KEXEC=false/' $FILESYSTEM_ROOT/etc/default/k
 ## Remove sshd host keys, and will regenerate on first sshd start
 sudo rm -f $FILESYSTEM_ROOT/etc/ssh/ssh_host_*_key*
 sudo cp files/sshd/host-ssh-keygen.sh $FILESYSTEM_ROOT/usr/local/bin/
-sudo cp -f files/sshd/sshd.service $FILESYSTEM_ROOT/lib/systemd/system/ssh.service
+sudo mkdir $FILESYSTEM_ROOT/etc/systemd/system/ssh.service.d
+sudo cp files/sshd/override.conf $FILESYSTEM_ROOT/etc/systemd/system/ssh.service.d/override.conf
 # Config sshd
 # 1. Set 'UseDNS' to 'no'
 # 2. Configure sshd to close all SSH connetions after 15 minutes of inactivity

files/sshd/override.conf

@@ -0,0 +1,4 @@
+[Service]
+ExecStartPre=
+ExecStartPre=/usr/local/bin/host-ssh-keygen.sh
+ExecStartPre=/usr/sbin/sshd -t