tests/functional: Convert most Aspeed machine tests

tests/functional: Convert most Aspeed machine tests

This is a simple conversion of the tests with some cleanups and
adjustments to match the new test framework. Replace the zephyr image
MD5 hashes with SHA256 hashes while at it.

The SDK tests depend on a ssh class from avocado.utils which is
difficult to replace. To be addressed separately.

Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>