test/py: make crash detection more robust

test/py: make crash detection more robust

test/py contains logic to detect the target crashing and rebooting by
searching the console output for a U-Boot signon message, which will
presumably be emitted when the system boots after the crash/reset.

Currently, this logic only searches for the exact signon message that
was printed by the U-Boot version under test, upon the assumption that
binary is written into flash, and hence will be the version booted after
any reset. However, this is not a valid assumption; some test setups
download the U-Boot-under-test into RAM and boot it from there, and in
such a scenario an arbitrary U-Boot version may be located in flash and
hence run after any reset.

Fix the reset detection logic to match any U-Boot signon message. This
prevents false negatives.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>