scripts: zephyr_flash_debug: flash like bossa-flash.sh

Add support for flashing targets compatible with bossa-flash.sh.

This is something of a bug-for-bug reimplementation, as the existing
flashing script makes some potentially unsafe assumptions.

Only tested by comparing commands that would be run.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>