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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

If a port cannot be identified (i.e. can not be associated with a board) the current version of the CLI requires to specify a board with the -b flag. Previously this wasn't necessary and the monitor would sometimes fail to work because it may require special dtr / rts settings.

This PR now reverts the old behavior that is more convenient but also adds a warning to the user when the board can not be identified and the default settings may not work as expected.

What is the current behavior?

$ arduino-cli board list
Port Protocol Type Board Name FQBN Core
/dev/ttyS4 serial Serial Port Unknown
$ arduino-cli monitor -p /dev/ttyS4
Please specify an FQBN. The board on port /dev/ttyS4 with protocol serial can't be identified
$ arduino-cli monitor -p /dev/ttyS4 -b esp32:esp32:esp32
Connecting to /dev/ttyS4. Press CTRL-C to exit.
^C

What is the new behavior?

$ arduino-cli board list
Port Protocol Type Board Name FQBN Core
/dev/ttyS4 serial Serial Port Unknown
$ arduino-cli monitor -p /dev/ttyS4
Using generic monitor configuration.
WARNING: Your board may require different settings to work!
Monitor port settings:
 baudrate=9600
 bits=8
 dtr=on
 parity=none
 rts=on
 stop_bits=1
Connecting to /dev/ttyS4. Press CTRL-C to exit.
^C
$ arduino-cli monitor -p /dev/ttyS4 -b esp32:esp32:esp32
Using default monitor configuration for board: esp32:esp32:esp32
Monitor port settings:
 baudrate=9600
 bits=8
 dtr=off <-- notice the esp32 requires DTR=off
 parity=none
 rts=off <-- and also RTS=off
 stop_bits=1
Connecting to /dev/ttyS4. Press CTRL-C to exit.
^C

Does this PR introduce a breaking change, and is titled accordingly?

No

Other information

Fix #2638