Merge pull request #2313 from jsmeix/cleanup_and_enhance_320_migrate_…

…network_configuration_files_issue2310

Cleaned up and enhanced 320_migrate_network_configuration_files.sh
that was triggered by #2310
plus some bugfixes where the old code (as far as I can understand it)
did not work. It should not change how it should have worked before
so it is primarily meant as a precondition to implement things like
#2312
in particular as first step a simple and generic basic method
for network setup at the end of "rear recover" via a new array
TARGET_SYSTEM_NETWORK_SETUP_COMMANDS