There previously was a mixup between a variable named `RECALBOX_VERSION`
and another one named `RECALBOX_VERSION_LABEL`. They were doing the same
thing but were scoped to different areas. This commit streamlines them
to use `RECALBOX_VERSION` everywhere.

The `recalbox.version` file is overwritten by this value at compile
time, thus we don't need it in the codebase. This commit removes and
git-ignores it.

This mixup led to a bug where if unset, `RECALBOX_VERSION` had no
default value, and `recalbox.version` was emptied at compilation time.
Fortunately, that only impacted local builds, since CI always defines
`RECALBOX_VERSION`. This commit fixes `RECALBOX_VERSION` default value
to be `development` on local builds.