yupdate - allow patching the D-Installer

[lslezak]

Problem

The current yupdate script can be only used from inst-sys for patching the standard YaST modules. It would be nice to extend it so it could be used the same way with the D-Installer. This would make testing and development of new features easier.

Solution

Adapt the script so it can work in a different environment (live medium).

• Check for the live medium environment (the root uses the overlay file system)
• Do not try to modify /etc/install.inf if it does not exist (it is not present on the live medium)
• Add pre and post hooks so some actions can be done before patching the system (installing some RPM packages) or after (restarting the DBus service). It runs the .yupdate.pre and .yupdate.post scripts when present in the source code.

Testing

• Updated unit test
• Tested manually

[coveralls]

Coverage: 40.77% (-0.02%) from 40.786% when pulling a9e06fb on d-installer_support into 88bc3ba on master.

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