The arm-trusted-firmware/default.nix expression exposes
`buildArmTrustedFirmware` and its `version?"2.7"` field to
`top-level/all-packages.nix`. Unfortunately it doesn't work.
Changing the version field doesn't change what version of the ATF
source code is used. Attempting to "lock" an installation to a
specific version by overriding this field (e.g. version="2.7") won't
work either; when nixpkgs bumps the version to 2.8 the user will end
up building the 2.8 source code but the resulting expression will be
labeled misleadingly in the store:
```
/nix/store/eeee...-arm-trusted-firmware-2.7/
```
**using the 2.8 source code**. So not only does `version` not lock
the version, it will actually *conceal* the fact that the underlying
source code has been upgraded!
Let's just remove the `version` field. It doesn't work and never did.
https://github.com/NixOS/nixpkgs/pull/185004#discussion_r939526830
Co-authored-by: Sandro <sandro.jaeckel@gmail.com>
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