machine.board: Introduce .board property

[Rahix]

Add a new protocol for all machines which were "derived" from a board.Board at some point. These should all implement the BoardMachineBase protocol which demands a .board property for accessing the original board.

This is useful because the board.Board machine might be used to define some parameters for this specific physical hardware, like an IP address. A BoardUBoot or BoardLinux machine would then want to be generic over these parameters. To illustrate:

class MyBoard(..., board.Board):
    ip_address = "1.2.3.4"

@tbot.testcase
def foo_test():
    with tbot.ctx.request(tbot.role.BoardUBoot) as ub:
        ip_address = ub.board.ip_address

    with tbot.ctx.request(tbot.role.BoardLinux) as lnx:
        ip_address = lnx.board.ip_address

One could now define different configs with different settings while using the same BoardUBoot and BoardLinux implementations with all of them.