Merge remote-tracking branch 'nl6720/fix-unbound-variables-in-_validate_options'

* nl6720/fix-unbound-variables-in-_validate_options:
  mkarchiso: error out of iso and netboot build modes if no boot modes are specified
  mkarchiso: split out build mode specific checks from _validate_options to _validate_requirements_buildmode_*
  mkarchiso: fix unbound variable errors in _validate_options
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