west: runners: Add support for multiple device IDs

In order to enable the use case where the underlying flash tool supports
bulk-flashing using multiple device IDs, augment the core runner class
with this new runner capability and implement it in the nrfutil runner,
since the nrfutil tool supports it natively.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>