runners: jlink: rework --dt-flash handling

Defer loading .config until we really need it, when we are flashing a
binary. Pre-emptively loading it is wasted effort if we're flashing a
.hex, which has been the default behavior when possible since
dcaabb860f ("west: runners: jlink: prefer .hex over .bin").

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