[Firmware] added functions to send debug messages

-Added a function comm_SendDebugMessage(), to send a text message to the
computer. This is useful for debugging.
-Added a function comm_SendDebugMessageDecimated(), to send a text message
to the computer, but only once out of N. Associated macros make the call
very convenient and in one line only (no needd for static int i=0; if(i++
% N == 0) commSendDebugMessage();).