The command now return all keymaps that's buildable for a
keyboard/revision. If the base directory of a keyboard does not contain
a 'rules.mk' file, nothing is returned. If the base directory contains a
'keymaps' directory, those keycaps will be returned for every revision.
`list_keyboards` replicates the `make list-keyboards` by globbing for all paths
that include `rules.mk` and then removing the paths that include `keymaps`.
This basis of this cli command could be reused in the future as a util, but is
not done so here since this would be the only place that would use it currently
Resolves#6911
