cmake/flash: persist python runners state in YAML

The YAML contents mirror the values in the ZEPHYR_RUNNER_CONFIG
variables, but they are phrased in terms of command line arguments.

This makes it possible for Python to intermix them with
runner-specific arguments, which is a step towards being able to set
arguments like --bin-file via board_set_runner_args(). The next step
is to handle them in Python too.

Move the RUNNERS_VERBOSE setting closer to its use while at it, to
preserve readability.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>