unify all compiled C++ into 1 binary file

[2bndy5]

should resolve #61

[2bndy5]

SERIAL_DEBUG is used to enable debug output in both RF24 and RF24Network libs. Because these changes compile both libs into 1 binary, we have no way to independently control which lib outputs debug statements. Both RF24 and RF24Network will output debug statements if we do

export CMAKE_ARGS=-DSERIAL_DEBUG=1
pip install -v .

Previously, I was able to control which lib's debug output is enabled when compiling the libs into separate binaries. I know this isn't a big concern, but the RF24 debug output can be noisy and make it difficult to see/find debug output from RF24Network.