handle new emergency button for red exchange mechanism

Rexm has now an emergency stop button to immediatly power down the motors.

They're will be new keywords to track:

• state of emergency stop button(pushed or released), 
• state of emergency flag  ( the button was pushed,  the button was not pushed)

In addition, there will be a new command to reset that flag

For the implementation, using the state machine is probably the best option.