runners: bossac: add --delay option

Add an escape hatch so users can insert a customizable delay
between asking the board to reboot into bootloader mode and
trying to program the new image. This lets people account for
OS-specific enumeration time done after the board resets.

Fixes: #56540
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>