arm-trusted-firmware: unfree only if hdcp.bin used; otherwise delete it

The `unfreeIncludeHDCPBlob` parameter was introduced as a result of
this reviewer request:

  https://github.com/NixOS/nixpkgs/issues/148890#issuecomment-1032002903

The default value `unfreeIncludeHDCPBlob?true` causes a change in the
`meta.license` field for all of the subpackages within
`pkgs/misc/arm-trusted-firmware/`, and results in them needing
`NIXPKGS_ALLOW_NONFREE=1`.

For non-Rockchip platforms the file hdcp.bin does not get included in
the output; the blob is for a Synopsys HDCP core that is currently
used only by Rockchip (although other companies could license it from
Synopsys in the future). Therefore on non-Rockchip we can delete
hdcp.bin before building instead of changing the license. This
preserves the ability to build them without NIXPKGS_ALLOW_NONFREE=1.

Let's do that.

Deleting hdcp.bin ensures that we won't be caught by surprise if some
future non-Rockchip Arm CPU licenses the same Synopsys HDCP core that
Rockchip is using.

Use easier-to-follow names for controlling the blob
inclusion/exclusion.  Also, if the blob is believed to be unnecessary,
delete it beforehand so we will know if we were wrong about that belief.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

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