Add support for the "Online Update"

Add support for the "Online Update"

"Online Update" is a new type of OS Update.
After "reboot fota" the normal boot will be performed, and in addition
the following targets will be run:

* online-update-pre.target - this target startup is reached very realy -
  after mouting the file system and before starting services. Within
  this target, changes to permission and directory structure, for
  example, should take place.

* online-update.target - this target is started after the basic services
  (like dlog, dbus) are started. Data migration of services should take
  place within this target.

If a service requires data migration after an upgrade, it should provide
a service that will perform the migration at the appropriate time of
system startup. For example:

  [Unit]
  Description=Bluetooth upgrade service
  ConditionKernelCommandLine=bootmode=fota
  Before=online-update.target bluetooth-share.service
  After=opt.mount
  DefaultDependencies=no

  [Service]
  Type=oneshot
  ExecStart=/usr/share/upgrade/scripts/bluetooth-share/bluetooth-share_upgrade.sh
  SmackProcessLabel=System

  [Install]
  RequiredBy=online-update.target

The lines:

  Before=online-update.target bluetooth-share.service
  After=opt.mount

are responsible for starting the service at the right time.

The old upgrade type is still supported. If the system is booted like
this:

    /usr/lib/systemd/systemd --unit=system-update.target

the old upgrade (Offline) will be performed. And if it runs like this:

   /usr/lib/systemd/system --unit=online-update.target

then the new upgrade (Online) will be performed.

Change-Id: I691af6c500da66fb537fd744e40fec2e7a607242