[File] Detect YAFFS images

[FC Stegerman]

* Christos Zoulas <christos at zoulas.com> [2023-02-03 20:19]:
> Committed, please test.

The test nesting in the big endian version seems to have gotten messed
up when converting to using a named entry (which I should have thought
to use myself); patch attached.

- FC
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