scripts: runners: nrfutil: Add a new external memory config param

This acts in a similar way to nrfjprog's --qspiini parameter introduced
in 7eb364b0, but works slightly differently in nrfutil: it requires a
config file in JSON format instead of .ini, and it is passed along to
the `execute-batch` command instead of providing it together with the
`program` command.

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