altos: Don't init packet slave on TD. Make slave start optional

altos: Don't init packet slave on TD. Make slave start optional

Oops. TeleDongle was starting the packet slave code, which kinda
wrecked its ability to receive telemetry packets. This patch simply
removes the packet slave code from teledongle as it cannot be used
(yet), it also makes the packet slave code initialization take a
parameter which controls whether to start that by default; in the
future, perhaps TeleDongle will gain a command to start packet slave mode.

Signed-off-by: Keith Packard <keithp@keithp.com>