configure: clean-up container cross compile detect

configure: clean-up container cross compile detect

The introduction of podman support inadvertently broke configure's
detect of the container support as the configure probe didn't specify
an engine type. To fix this in docker.py:

  - only (re)set USE_ENGINE if --engine is specified
  - enhance the output so docker is no longer just yes

In the configure script we can at least start cleaning up the
detecting and naming of variables. To avoid too much churn the
conversion of the various make DOCKER_foo variables has been left for
future clean-ups.

Fixes: 9459f754134b
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: David Hildenbrand <david@redhat.com>