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I see that, on linux, the library searches in ~/.config/.... and /etc/xdg/....

~/.config/arduino15 may have been a valid alterantive if used from the beginning but since we used ~/.arduino15 to store everything I'd like to continue to use that one. Also /etc/xdg seems a bit overkill to me, why the extra xdg sub-folder? /etc/arduino-cli.conf seems enough...

Let's implement here some ideas from: #30
To recap some points:

1. We should look for a configuration in some predefined places, for example on Linux:

• /etc/
• ~/.arduino15/
• ${pwd}

those directories are listed in increasing order of priority and any config file found should override the previous one (I mean the options defined in ${pwd} should take priority on the ones in /etc for example).

2. we can navigate parent folders for ${pwd} to search for a config file, this would allow to make a per-project configuration.

and this is added by me:

3. we should consider a better future-proof name for the config file like: arduino-cli.conf so we have a nice /etc/arduino-cli.conf instead of /etc/.cli-config.yaml

Do we need to worry about $arduino_ide_dir/portable, as described in https://www.arduino.cc/en/Guide/PortableIDE ? I don't use ~/.arduino15, because I keep multiple versions of the IDE around for testing, and I don't want one version of the IDE clobbering the files and configs of another version.

@bxparks we pushed forward portable support here: #123

Also we started bundling the cli in the official Beta IDE, at the moment arduino-cli detects if it's bundled with the IDE and also if there is a portable folder:

~/Code/arduino/build/linux/work$ ls -l
total 29272
-rwxr-xr-x 1 cmaglie cmaglie 882 feb 1 10:05 arduino
-rwxr-xr-x 1 cmaglie cmaglie 13822120 feb 1 10:05 arduino-builder
-rwxr-xr-x 1 cmaglie cmaglie 16000485 feb 1 10:05 arduino-cli
-rwxr-xr-x 1 cmaglie cmaglie 5161 feb 1 10:05 arduino-linux-setup.sh
drwxr-xr-x 13 cmaglie cmaglie 4096 feb 1 10:05 examples
drwxr-xr-x 4 cmaglie cmaglie 4096 feb 1 10:05 hardware
-rwxr-xr-x 1 cmaglie cmaglie 9796 feb 1 10:05 install.sh
drwxr-xr-x 4 cmaglie cmaglie 4096 feb 1 10:05 lib
drwxr-xr-x 21 cmaglie cmaglie 4096 feb 1 10:05 libraries
drwxr-xr-x 6 cmaglie cmaglie 4096 feb 1 10:05 reference
-rw-r--r-- 1 cmaglie cmaglie 87826 feb 1 10:05 revisions.txt
drwxr-xr-x 4 cmaglie cmaglie 4096 feb 1 10:05 tools
drwxr-xr-x 4 cmaglie cmaglie 4096 gen 3 17:15 tools-builder
-rwxr-xr-x 1 cmaglie cmaglie 86 feb 1 10:05 uninstall.sh
~/Code/arduino/build/linux/work$ ./arduino-cli config dump
proxy_type: auto
sketchbook_path: /home/cmaglie/Workspace/sketchbook-cores-beta
arduino_data: /home/cmaglie/.arduino15
board_manager:
 additional_urls:
 - http://arduino.esp8266.com/stable/package_esp8266com_index.json
~/Code/arduino/build/linux/work$ mkdir portable
~/Code/arduino/build/linux/work$ ./arduino-cli config dump
proxy_type: auto
sketchbook_path: /home/cmaglie/Code/arduino/build/linux/work/portable/sketchbook
arduino_data: /home/cmaglie/Code/arduino/build/linux/work/portable
board_manager: null

BTW we barely tested the "portable" scenario, (for example now I noticed the board_mager: null in the config dump) so it may require some more work.

In this latest batch of commits I implemented the following:

we can navigate parent folders for ${pwd} to search for a config file, this would allow to make a per-project configuration.