List subdirectories, and fix root listing, on embedded filesystems

[meeloo]

Two separate reasons a client saw an empty root on a USE_EMBEDDED_FILES build.

EmbeddedFiles::FindFirst only ever reported files, so directories were invisible even when they existed and could be navigated into directly. It now walks the directory list first and reports immediate children with isDirectory set, then the files.

It also disagreed with its own caller about how to spell the root. MassStorage::DirectoryExists truncates a trailing separator before delegating, so the root arrives as "", while MassStorage::FindFirst passes the path through untouched, so the same request arrives as "/". The empty-root test matched only the first spelling, and "/" matched no directory entry either, so the listing came back empty rather than failing - which is why it looked like "there is nothing there" instead of "the lookup is wrong". FindFirst now normalises the trailing separator itself.

Duet3_MB6HC is unchanged in size; this code is only compiled for embedded-filesystem builds.

Testing note: nothing upstream builds USE_EMBEDDED_FILES, so this cannot be exercised without a build config for the embedded variant. I have one at meeloo/RepRapFirmware:build/embedded-mb6hc-config and can open it as a separate PR, fold it into this one, or leave it out - whichever suits

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[meeloo]

I have read the Duet3D CLA v2.0 and I hereby sign it