monitor: use espflash configuration

[romainreignier]

When using the monitor subcommand, two questions are asked by espflash:

✔ Use serial port '/dev/ttyACM0' - USB JTAG/serial debug unit? · yes
✔ Remember this serial port for future use? · yes

thread 'main' panicked at /home/rre/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/espflash-3.3.0/src/cli/config.rs:159:48:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

If yes is answered to the question: "Remember this serial port for future use?" the program panic because the save_path variable of the espflash Config is not initialized to a valid directory to store a persistent configuration.

Using the load() function allows to skip the questions on next use.

[ivmarkov]

It skips the questions but what happens in addition?
Is it creating some sort of espflash.toml hidden somewhere? If yes, I'm not sure we actually want that? We probably want - if the user had not specified a serial port in the espfactory.toml conf - that the user is asked about the port every time, and moreover, the user should not be asked about "shall I save this port for next time".

[romainreignier]

If the user has an espflash config file in the current or parent Dir or $HOME/.config/espflash then the questions are skipped and the found USB ids are used.
Otherwise, if yes is answered to the second question, a new file is created in $HOME/.config/espflash.
Skipping the second question will need some changes in espflash I think.

[ivmarkov]

Sorry for the delay here. Will follow up over the weekend.