runners: core: try to improve --dt-flash option help
runners: core: try to improve --dt-flash option help

This option has existed since the beginning of the runners package,
which greatly predates the way DT is used in zephyr right now. It
never really worked the way I wanted it to but it's too much work to
fix it now. Try to improve the help a bit at least while I'm looking
at it again.

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr