west: runners: nrf: Add an option to control the ext erase mode

The erase mode for any external memory that is mapped to the address
space of the MCU is derived from the internal non-volatile memory erase
mode. In order to allow users to override the default value, add a new
--ext-erase-mode command-line option that takes an erase mode just like
--erase-mode does.

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