Two Flatcar versions were used in /etc/portage/make.conf both in the SDK
and in the boards.
Use only a single version by default to get the expected results and not
something else when using binary packages.
The Rust crossdev package was never uploaded to /sdk/ and always
had to be compiled again.
Upload it in a separate toolchain-arm64 directory because /Packages in /crossdev/
doesn't refer to the Rust package and its use flags.
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