Many of the platforms are similar enough to other platforms that they'll
share the same BIOS files.
The current storage method stores BIOS files per platform, meaning that
files can be duplicated multiple times to satisfy the requirements of
each platform.
This change stores the files as their hash with a .bios extension
(example: `85ad74194e87c08904327de1a9443b7a.bios`) in a flat directory
structure. This allows BIOS files that are used by multiple platforms to
be shared without duplication.
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