Make receiver value printing commands

[stenbergd]

To test that the receiver behaves correctly, implement commands to print the following:

getreceiver - prints all receiver channels' current value

startreceiversampling: - Starts a thread that prints the receiver value every second for seconds

(NOTE: Be careful not to let the program create/kill multiple threads since they will clutter RTOS heap)

getreceivercalibration - Prints the receiver calibrated values

[stenbergd]

getreceiver done.

[stenbergd]

get receiver calibration values done.

[stenbergd]

The start receiver sampling is mostly done

Need stop receiver sampling command

Needed to use heap2.c instead so we can create/delete task

Also small bug when printing sample time and duration

[stenbergd]

Also, we must look more at strncat to avoid overwriting string output buffer. The size sets only how many bytes we are allowed to append to the end of input string and does not take heed of max size of string.

[stenbergd]

Fixed the "small bug" when printing sample time and duration