Release 8.1.0 (and newer) broke on custom board without USB

[MSmeets94]

We have a custom board without USB using release 8.1.0. Because we don't have USB, we build with 'build_unflags = -DUSBCON'.

platform-atmelsam targets framework-arduino-samd ~1.8.13 which was never a problem until 1.8.14 came out.

Since the release of framework-arduino-samd 1.8.14, a LineInfo declaration was added to the Serial_ class in USBAPI.h which causes a compilation error because the definition of LineInfo is in CDC.h (which is not defined because of the unbuild flag).