[board] Add RTT for boards without on-board serial logger

[salkinium]

A lot of older STM32 discovery boards have an STLinkv2 on board, but did not route the serial link to it.
Smaller boards off Aliexpress do not even have an on-board debug probe or a serial port.

This adds a RTT based version of the board logger by default.
Contrary to board loggers backed by serial devices, the RTT may never be read, therefore it cannot be blocking, otherwise any fiber that logs would eventually always halt. Therefore these board loggers discard data on overflow.

I did not add a RTT logger to devices with a USB bootloader, as they are not expected to be used with an external debug probe.

• Tested in as much hardware as possible.
• Fixed wrong default of RTT receiver buffer. Unlike the UART drivers, which can store at least one byte (often more) in hardware without software buffer, a RTT buffer size of zero means nothing can be sent.
• Fixed deprecated use of hla_swd for STLinks.
• Remove conflicting logger examples.