This update for checkmedia fixes the following issues:

- Fix compatibilty issue with older gcc compiler.
- work also wi
- Add support for signed media. (bsc#1139561)
- adjust tagmedia script
- Update documentation.
- Allow to set specific gpg key for signature verification.
- Added tests for signature verification.

This update was imported from the SUSE:SLE-15-SP1:Update update project.