Implementation of a Board-based flow (WIP)

[cmaglie]

This PR tries to implement a board-centric flow like the following:

$ arduino-cli board install zero
....
$ arduino-cli upload -b zero
...

In the examples above zero is a board Alias. Each board alias corresponds to a FQBN (for example zero corresponds to arduino:samd:arduino_zero_edbg). There should be a table that maps Alias and FQBN 1:1 but how this is obtained is still TBD.

• A board alias can be used anywhere a FQBN was previously expected. The old flag --fqbn is still accepted but has been deprecated in favor of --board/-b that can accept both alias and FQBN.
• FQBNs may carry also configuration options for a board. To allow configuration using aliases an extra flag has been added --board-config/-c that can accept a config option. The flag may be used multiple times. For example:
  arduino-cli board details arduino:avr:nano:cpu=atmega168
  can now be rewritten as:
  arduino-cli board details nano -c cpu=atmega168
  or:
  arduino-cli upload -b arduino:avr:nano:cpu=atmega168
  can now be rewritten as:
  arduino-cli upload -b nano -c cpu=atmega168

[cmaglie]

Too much time passed, and I'm closing this one until the board-based flow will regain traction.