Send heartbeat to start MAVLink

[Acumane]

Send a heartbeat so cdcacm_autostart starts MAVLink. Necessary on some flight controllers if you don't explicitly run mavlink start

INFO  [cdcacm_autostart] Starting CDC/ACM autostart
...
INFO  [cdcacm_autostart] /dev/ttyACM0: launching mavlink (HEARTBEAT v1 from SYSID:255 COMPID:0)
INFO  [mavlink] mode: Onboard, data rate: 100000 B/s on /dev/ttyACM0 @ 2000000B

Adds --poke to demo CLI. Works for me

[Acumane]

Tested two EchoMAV EchoPilots on both Linux and Mac