Make uart more robust - add start byte, add crc check.

[kareltucek]

Changes:

• decrease uart timeouts, but at the same time forgive one missed ping
• check consistency of uart messages
• start message frames explicitly, thus reducing issues with interference where a single spike can shift the entire message by a byte.
• reset the link protocol when an inconsistent uart message is received

This PR should solve:

• lags when the bridge cable is connected
• some of the key chatter and stucking

(Reseting the entire link is hardly ideal, and later should be replaced by a more robust solution.)