[abi] allow to pass variables by value

before the generated ABI callbacks always had a signature with `const type *var` where type was e.g float.
Now the generated signature is simply `type var`.
To pass const pointers again, set it accordingly in abi.xml, e.g. type="const float *" instead of type="float"

sw/airborne/boards/ardrone/navdata.c

@@ -550,7 +550,7 @@ void navdata_update()
         // Check if there is a new sonar measurement and update the sonar
         if (navdata.ultrasound >> 15) {
           float sonar_meas = (float)((navdata.ultrasound & 0x7FFF) - SONAR_OFFSET) * SONAR_SCALE;
-          AbiSendMsgAGL(AGL_SONAR_ARDRONE2_ID, &sonar_meas);
+          AbiSendMsgAGL(AGL_SONAR_ARDRONE2_ID, sonar_meas);
         }
 #endif