Current behavior

When no cores are installed, I would expect that listall returned all the existing boards (like the Boards menu in the IDE). For a new user it's not clear that it only lists the boards whose cores are installed, also because core install is documented in the README after board listall.

$ arduino-cli board listall
Board Name FQBN

$ arduino-cli board
[...]
  listall     List all known boards and their corresponding FQBN.
[...]

Suggested behavior

$ arduino-cli board listall
No known boards: install your first core with "arduino-cli core install <core>" to support its boards.
$ arduino-cli board
[...]
  listall     List all known boards (and their corresponding FQBN) according to the installed cores.
[...]

Also, would it be possible to add a board search command to list all the existing boards (even if not installed)? Do we have such data around?
Most users think like this: I have a board -> I know its name but I don't know what core it requires -> Please install whatever core is needed for my board

Additional context

Additional requests

• Explain better that "board listall" depends on installed cores #550 (comment)
• Explain better that "board listall" depends on installed cores #550 (comment)