Out of tree hardware platforms

[Duss Pirmin]

Hi Genodians

I'm enabling base-hw on the IoT gateway of CompuLab [1].

I do this in a separate repository in the same way as arndale, panda etc
are done in genode-world. This works well apart from some minor
modifications on the imx8q_evk (for which I will create an issue as soon
everything is finalized and cleaned up).

The only thing I'm struggling with is that /tool/run/boot_dir/hw isn't
used. This leads to the error

unknown platform no linker address known

If I add the platform to genode/tool/run/boot_dir/hw building works. But
as with the other out of tree platforms I'd like to keep the platform
out of the Genode repository, for cleanness.

The repository is linked to genode/repos/... and added to REPOSITORIES
in build/arm_v8a/etc/build.conf.

[1]
https://www.compulab.com/products/iot-gateways/iot-gate-imx8-industrial-arm-iot-gateway/

Best regards,
Pirmin