serial: make LCR, modbus, SCPI over serial depend on generic serial comm

Add guards around the implementation of ES51919 chip support for LCR, as
well as modbus and SCPI over serial. To accept when the source files get
compiled in the absence of their dependencies, end up with an empty
implementation in that case.

This approach can simplify build rules when several optional external
dependencies result in differing sets of supported communication means.