tests/avocado/machine_aspeed: Fix the broken ast2[56]00_evb_sdk tests

tests/avocado/machine_aspeed: Fix the broken ast2[56]00_evb_sdk tests

test_arm_ast2500_evb_sdk and test_arm_ast2600_evb_sdk are currently
failing. The problem is that they are trying to look for the login
prompt that does not have a newline at the end - but the logic in
_console_interaction() only handles full lines. It used to work by
accident in the past since there were sometimes kernel (warning and
error) messages popping up that finally provided a newline character
in the output, but since the tests have been changed to run with the
"quiet" kernel parameter, this is not working anymore.

To make this work reliably, we must not look for the "login:" prompt,
but have to use some text ending with a newline instead. And in the
ast2600 test, switch to ssh instead of trying to log into the serial
console - this works much more reliable and also has the benefit of
excercising the network interface here a little bit, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20230421110345.1294131-3-thuth@redhat.com>
[AJB: remove stray debug log]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230424092249.58552-10-alex.bennee@linaro.org>