UnbufferedSerial: Replace DirectSerial, RawSerial classes

[hugueskamba]

Description

Remove DirectSerial and deprecate RawSerial. Instead
Promote UnbufferedSerial as it provides unbuffered I/O
implementing features from DirectSerial and RawSerial.
It adds FileHandle features to RawSerial and add
add SerialBase features to DirectSerial.

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

Release Notes

[hugueskamba]

@evedon Please review 310a7cd

[evedon]

I find confusing that the serial test is mixed up with the greentea test infrastructure. Ideally, the serial test on the target side should not need to know about the details of the greentea protocol. But we need to share the serial line between the greentea test infrastructure i.e. the “control path” and the actual test i.e. “the data path”. Unfortunately there is no simple way (I believe) to allow the host test to send/receive raw data; only the send_kv and parse_kv functions are provided.
So I am accepting that the test needs to use the greentea protocol. I’d like that we limit the dependency on the protocol to the minimum and more importantly that we avoid code duplication.
I will propose some suggestions in the PR review.

[hugueskamba]

Raising a PR upstream as this was approved.