Refactor upgrade test for generic file server

[troglobit]

The upgrade test currently has a FileServer class that could be refactored into a generic class for other tests. This would allow Infix DUTs to download whatever they want from "the Internet", e.g., a too big container image, or customer specific test data.

For this to be properly versioned/integrated and reproducible (think: make sdk), the big container image, or customer specific test data, would be handled by a generic package/foo/foo.mk (not host package!) that employs Buildroot's download and hash verification before installing the data, not to output/target/, but to $BINARIES_DIR, which the test could then map into the Infamy docker and serve as a file area.

[troglobit]

CCB decision: wkz says, remember to restore boot order after test. (Test system relies on netbooting)

[mattiaswal]

This is done in #846