Update service dependencies to start on early boot

This changes log2ram dependencies to start as early as possible, before other
services that may require access to /var/log, including rsyslog and
systemd-journald.

Additional dependencies on shutdown.target are required to ensure that
the service will be stopped at shutdown/reboot when DefaultDependencies
is set to "no" [1].

RequiresMountsFor declares dependency on /var mount when it is a mount point.

IgnoreOnIsolate prevents stopping the service when isolating another unit.

[1]: https://www.freedesktop.org/software/systemd/man/systemd.service.html

Remove log2ram log on startup.

Sync the ramlog back to disk, hourly.

* Write sync status to HDD_LOG/log2ram.log.

- Store sync status directly in the HDD log, to make sure we know
  about it if something fails.  This won't help if the HDD_LOG
  variable is borked, but it'll help with everything else.

- Made sync a function to avoid repeating five lines three times.

- log2ram now requires (cp or rsync) and tee, mount, umount.

* Split sync into syncFromDisk and syncToDisk.

- After restarting, I realized that I had combined both types of sync
  into one line, when the syncing failed.  This fixes that bug.

* Exclude log2ram.log from rsync so rsync doesn't delete it on sync.

- Since the cron job could be called before the user reboots after
  install (or if log2ram is disabled), we check that the hdd log
  location exists before syncing.

- Copy the log2ram.hourly sync script to cron's hourly directory on
  install.

- Store sync status directly in the HDD log, to make sure we know
  about it if something fails.  This won't help if the HDD_LOG
  variable is borked, but it'll help with everything else.

- Made sync a function to avoid repeating five lines three times.

- log2ram now requires (cp or rsync) and tee, mount, umount.

From @datenklause
With Type=oneshot, systemd waits until the start action has completed
before it considers the unit to be active, so dependencies start only
after the start action has succeeded. With Type=simple, dependencies
will start immediately after the start action has been dispatched.

thank hardvk0 from armbian forum

no x flag for logr2ram.server

The name of the mount point is now log2ram

Move "Before" to "[Unit]"

Update README.md to correct path to log2ram

This tweak is required to make it run under Ubuntu 16.04