Support a `make debug` target

[dannystaple]

It would be helpful to add a make target like make debug, which will set debug=1 so there is a serial dump of activity, and preferably write out the firmware to a filename like NINA_W102-debug-1.7.4.bin.

This probably requires sketch.ino.cpp using a preprocessor macro for the debug variable instead of a variable, and some overriding of CFLAGS/CPPFLAGS in the make target.