Float and better 3rd party library compatibility (#2534)

* define as float (not double)

* Avoid #define of 1 or 2 char symbols

Having this file define 'A' and 'C' pollutes
the global namespace, and causes conflicts
with other libraries that also pollute the
global namespace with short #defines.
It's easier to fix this header.

* unused variable warning

wled00/wled_serial.cpp

@@ -46,8 +46,6 @@ void handleSerial()
   static byte red   = 0x00;
   static byte green = 0x00;
 
-  uint16_t nBytes = 0;
-  
   while (Serial.available() > 0)
   {
     yield();