Unlike custom banners which work as an override, this mechanism works as
a fallback. The use case is if you have games you don't really play but
want to keep around for testing purposes without filling up your NAND
with lots of saves. For ease of use, the directory structure is the same
but only title/$title_hi/$title_lo/data/banner.bin files are
relevant.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

It's bundled with the Wii saves for simplicity, since it's quite small.

Since all FS access will go through the new FS interface (PR #6421)
in order to keep track of metadata properly, there is no need to return
absolute paths anymore.

In fact, returning host paths is a roadblock to using the FS interface.

This starts the migration work by adding a way to get paths that are
relative to the Wii NAND instead of always getting absolute paths
on the host FS.

To prepare for future changes, this commit also makes returned paths
canonical by removing the trailing slash when it's unneeded.

Eventually, once everything has been migrated to the new interface,
we can remove the "from" parameter.

Also drops trailing underscores from said parameter names.