Use system toolchain on aarch64 for BusyBox

The toolchain downloaded when executing the build-scripts on aarch64 might be a
bare-metal so use the toolchain provided by the system.

Signed-off-by: Patrik Berglund <patrik.berglund@arm.com>
Change-Id: Ie25e785baca413d5503119370f0c27127fb7b64f