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all-packages.nix: expose arm-trusted-firmware
This commit exposes
```
arm-trusted-firmware = callPackage ../misc/arm-trusted-firmware { }
```
at the top level. Without this it is impossible for downstream
users like [ownerboot](https://git.sr.ht/~amjoseph/ownerboot) to
override options like `unfreeIncludeHDCPBlob` and dependencies like
`openssl`.