The problem with assert based implementation is that we are missing
crucial information like what was in output in error message. This just
let's pexpect throw exception and that is fine.

The standard is to use 120 columns but my editor was configured to 79 so
I modified it to be of correct width in this commit.

These new marks are intended to be used if specific resource is required
and such test would we run only if that resource was configured for that
target.

This moves logic of uboot handling to nsfarm from tests. Fixtures in
tests now only handle cleanup and call appropriate setup from nsfarm.

This is handy for debugging. It allows you to access serial console
during tests and that way developer can access directly state it
requires.
The way how it works is that developer is going to call:
board.serial_miniterm()
in appropriate location.

To make this work pytest have to be run without terminal redirection.
This is as easy as appending -s to pytest call.

This is initial documentation describing parts of NSFarm that are
already known and are expected to be exactly that way.