Provide consistent serial console interface for samples

[mikee47]

This PR provides a consistent mechanism for a basic serial console interface using the LineBuffer class.

Refactor LineBuffer

Implement LineBufferBase so we take non-trivial methods out of header.

Add bool, String operators, printTo method&

Simplify usage

Add process, processKey methods and update samples

Use LineBuffer for LiveDebug, Basic_Ota and Basic_FlashIP samples.
These new methods are used to simplify application code.

Note that the CommandProcessing::Handler class hasn't been updated here,
mainly because it requires customisation of EOL handling (setCommandEOL, getCommandEOL).
Presumably this is because of the variation in EOL sequences (CR, LF, CRLF or LFCR combinations).
The LineBuffer processKey method doesn't care, it handles all of these.

[slaff]

@mikee47 can you rebase?