misc_systetmd_units: Add service unit for logging over serial console

This service unit file was added to the system in June to be able to log Linux
Kernel crashes. Put it under version control.

```
$ ls -l --full-time /etc/systemd/system/serial-log@.service
-rw-r--r-- 1 root system 254 2017-06-23 10:24:55.000000000 +0200 /etc/systemd/system/serial-log@.service
```

install.sh

@@ -82,6 +82,7 @@ install_exec make-automaps/make-automaps          "$DESTDIR$usr_sbindir/make-aut
 install_data misc_systemd_units/automount.service "$DESTDIR$systemdunitdir/automount.service"
 install_data misc_systemd_units/gdm.service       "$DESTDIR$systemdunitdir/gdm.service"
 install_data misc_systemd_units/lazy-umount-nfs.service "$DESTDIR$systemdunitdir/lazy-umount-nfs.service"
+install_data misc_systemd_units/serial-log@.service "$DESTDIR$systemdunitdir/serial-log@.service"
 install_exec mxgrub/mxgrub                        "$DESTDIR$usr_sbindir/mxgrub"
 install_exec mxnetctl/mxnetctl                    "$DESTDIR$usr_sbindir/mxnetctl"
 install_exec mxrouter/mxrouterctl                 "$DESTDIR$usr_sbindir/mxrouterctl"

misc_systemd_units/serial-log@.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=TTY logger
+
+[Service]
+ExecStartPre=/usr/bin/stty -F /dev/%I 115200 -echo
+ExecStart=-/usr/bin/sh -c "/usr/bin/cat /dev/%I >> /var/log/%I.log 2>&1"
+Type=idle
+Restart=always
+RestartSec=0
+UtmpIdentifier=%I
+
+[Install]
+WantedBy=basic.target