west: runners: Add support for a common --reset argument

Some of the runners in the tree have been adding their own,
class-specific versions of a switch to instruct the runner to reset or
not the device after flashing.

In order to better support multi-image builds that require more than one
flash operation, introduce a new --reset,--no-reset command-line
parameter that is part of the RunnerCaps so taht this functionality can
be accessed in a standardized manner.

Implementations for the new parameter are provided for the runner
classes that were already configurable in this regard.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>