ota: Fix address argument of ota.block

There was a typo that caused it use argument 2 instead of 1 for the
address.
ota: Increase MAX_TRIES to 5

This seems to make things a bit more resilient.
ota: Add a small delay between retries of the same packet

It seems sometimes multiple packets fail in a burst, hopefully this
helps prevent a failed flash in those cases.
ota: Support 0xffff in ota.start

Previously, this would send all packets to the broadcast address,
causing mayhem when multiple scouts were in ota mode (random ones would
reply to the ping packets). Now, it broadcasts the initial ping, but
then saves the id of the scout that responded and directs further
traffic there.