"board listall" and "core search --additional-urls" don't seem to work after a fresh install via homebrew

[alranel]

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

[alranel]

After running arduino-cli core install arduino:samd, the output of arduino-cli board listall is not empty anymore. It was not clear to me that board listall depends on the installed cores (although it totally makes sense) because the README describes core install after board listall.
Maybe the description for board listall should mention that it lists the supported boards according to the installed cores and that if you don't see your board listed you should install its core. I'd put this hint also in the output for board listall when no entries are returned.

The issue about core search --additional-urls is still there though.

[alranel]

On the other hand, would it be possible to list all the known boards even if their core is not installed?
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

[alranel]

I opened a new issue for this in #550. The one about core search --additional-urls was partially fixed in #548