SITL shell: Do math using the shell

Bash or zsh from the last 10 years or so do math using the $((a + b)) syntax. This saves us from having bc as dependency.

ROMFS/px4fmu_common/init.d-posix/rcS

@@ -191,15 +191,15 @@ fi
 
 # Adapt timeout parameters if simulation runs faster or slower than realtime.
 if [ ! -z $PX4_SIM_SPEED_FACTOR ]; then
-	COM_DL_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 10" | bc)
+	COM_DL_LOSS_T_LONGER=$((PX4_SIM_SPEED_FACTOR * 2))
 	echo "COM_DL_LOSS_T set to $COM_DL_LOSS_T_LONGER"
 	param set COM_DL_LOSS_T $COM_DL_LOSS_T_LONGER
 
-	COM_RC_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 1" | bc)
+	COM_RC_LOSS_T_LONGER=$((PX4_SIM_SPEED_FACTOR * 2))
 	echo "COM_RC_LOSS_T set to $COM_RC_LOSS_T_LONGER"
 	param set COM_RC_LOSS_T $COM_RC_LOSS_T_LONGER
 
-	COM_OF_LOSS_T_LONGER=$(echo "$PX4_SIM_SPEED_FACTOR * 10" | bc)
+	COM_OF_LOSS_T_LONGER=$((PX4_SIM_SPEED_FACTOR * 2))
 	echo "COM_OF_LOSS_T set to $COM_OF_LOSS_T_LONGER"
 	param set COM_OF_LOSS_T $COM_OF_LOSS_T_LONGER
 fi