serial interface: add `std::ostream` adpater for Arduino `Serial`

[dhebbeker]

Similar to std::cout a global object shall be provided which allows to print formatted output to the serial interface using the Arduino Serial library. This is an interface adapter to 3rd party code.

A name could be serial::cout.

Acceptance criteria:

• a global object is available which fulfils the std::ostream interface
• that object can be used to write to serial interface like std::cout

See WIP.