test: Allow signaling that U-Boot is ready

test: Allow signaling that U-Boot is ready

When Labgrid is used, it can get U-Boot ready for running tests. It
prints a message when it has done so.

Add logic to detect this message and accept it.

Note that this does not change pytest, which still (also) looks for the
U-Boot banner. This change merely makes it possible for pytest to
believe Labgrid when it says that the board is ready for use.

In several cases, the board starts up and Labgrid receives some initial
output, then pytest starts and misses some of that output, because it
came in while Labgrid had the console open. Then pytest fails because
it doesn't see the expected banners.

With this change, Labgrid handles getting U-Boot to a prompt, in a
fully reliable manner. Then pytest starts up and can simply start
running its tests.

But, again, this does not prevent pytest from handling a banner if one
is provided (e.g. if not using the Labgrid integration).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>