Boards with an integrated debugger/programmer that also provides the
serial as UART <--> USB adapter, the TTY serial matches the serial of
the programmer.
This adapts the `serial.inc.mk` to set the `DEBUG_ADAPTER_ID` to the
TTY serial if (and only if) `MOST_RECENT_PORT` *and*
`DEBUG_ADAPTER_ID_IS_TTY_SERIAL` both have a value of `1`. Boards with
an integrated programmer are expected to set
`DEBUG_ADAPTER_ID_IS_TTY_SERIAL` to `1` in their `Makefile.include`.
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