scripts: runners: nrf: Honor the --erase flag for external memory

Both backends supported as runners for nRF ICs, nrfjprog and nrfutil,
support erasing external memory as part of the programming operation.
Before this patch, and when the firmware was detected to be partially or
fully placed in external flash by inspecting the .hex address range, the
runner would instruct the backend tool to fully erase the external
flash (but the nrfjprog runner would ignore that, always erasing only
the sectors required). Instead, correctly default to erasing only the
sectors that are required to program the new firmware image in both tools,
and erase it completely only when the --erase flag is provided by the user.

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