Emergency mode: add emergency-target-holder.service

Emergency mode: add emergency-target-holder.service

The emergency-target-holder.service prevents emergency.target from completing.
If emergency.target is not completed,
other services that are not intended can not be run in emergency mode.

Detailed information.
============================================================================================================================
1. If local-fs.target fails, emergency.target is started by 'OnFailure=emergency.target'.

2. By 'OnFailureJobMode=replace-irreversibly' option,
   emergency.target cannot be canceled until completion.

3. When starting a new service by any activation(dbus, path, socket) in emergency mode,
   sysinit.target is always checked and started by 'DefaultDependency=yes'.

4. sysinit.target stops emergency.target because of the 'Conflicts=emergency.target' setting.

5. However, emergency.target can not be stopped
   because it started with 'replace-irreversibly' and not finished yet.

6. So sysinit.target can not be started.

7. New service can not be run because sysinit.target could not be started.
============================================================================================================================

Logs when running new services after applying this patch
=============================================================================================================================
bash-3.2# systemctl start deviced.service
Failed to start deviced.service: Transaction for deviced.service/start is destructive (emergency.target has 'start' job queued, but 'stop' is included in transaction).
See system logs and 'systemctl status deviced.service' for details.

bash-3.2# systemctl start sysinit.target
Failed to start sysinit.target: Transaction for sysinit.target/start is destructive (emergency.target has 'start' job queued, but 'stop' is included in transaction).
See system logs and 'systemctl status sysinit.target' for details.
==============================================================================================================================

Change-Id: I2cefadd7228d463fe1755e0c475f4563d98c8260
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>