2.5s for tilt_emergency_millis might be too low

[alexvaut]

TILT_EMERGENCY_MILLIS value of 2.5s is too high. In some cases, the ground is not flat and the robot is moving very slowly there. We need to allow users to setup their own value.

Implementation: The only place I found where the firmware receives information is through ll_heartbeat structure where emergency flag can be reset. So I'm thinking to add a new field called tilt_emergency_millis, set in in mower_comms by reading an env var. The firmware will read and set it when it receives the heart beat. While I'm at it, I will probably set lift_emergency_millis as well. What do you think ?