This is required for the eventual removal of `$PORTDIR` and
`$PORTDIR_OVERLAY` and ensures toolchain rebuilds/updates with
`./build_packages --nousepkg` don't erroniously try to use ebuilds from
`/usr/portage` inside of the SDK.
In order to fix up the build_toolchains script the crossdev overlay
needs to be setup properly, previously only setup_board did it.
Overall silences a lot of warnings and fixes an issue with crossdev:
/usr/bin/emerge-wrapper: line 48: /eclass/toolchain-funcs.eclass: No such file or directory
/usr/bin/emerge-wrapper: line 49: tc-arch: command not found