fix motors.cpp and add functions to header file

hover-controller/motors.cpp

@@ -113,7 +113,7 @@ void motor_process(char*msg){
         case 'R':
             break;
         default:
-            eprintf("[motors] mode not valid (%c)\n", *servo_mode);
+            eprintf("[motors] mode not valid (%c)\n", servo_mode);
             strcpy(msg, "ESC_MODE_NOK");
             return;
     }

hover-controller/motors.h

@@ -4,5 +4,7 @@
 #include "servos.h"
 void motor_process(char*msg);
 void all_motors_stop();
+void all_motors_zero();
+void all_motors_start();
 
 #endif // __MOTORS_H__