Implementing additional SPECS in genode-world

[Johannes Schlatow]

This is a follow-up of my previous approach [1] on separating the support of additional platforms from the official repository:

In addition to the zynq_qemu SPEC variable that is already existent in the official repository, I'd like to add SPEC variables like `zynq_parallella` and `zynq_zedboard` to genode-world. However, I just ran into a conceptual problem that I'd like to discuss here.

First, I can place a core.mk in genode-world/lib/mk/specs/zynq_parallella, which is then used as soon as I change the SPECS from zynq_qemu to zynq_parallella. I am also able to set the include directories properly.

The point where I got stuck, however, is that I need to include the less-specific configuration files within base-hw, which I achieve by the following line:

	include $(call select_from_repositories,lib/mk/spec/zynq/core.inc)

Now, the issue is that including makefiles from other repositories won't work properly. This is due to the fact that $REP_DIR (used e.g. in core.inc) will evaluate to `[...]/repos/world` and not `[...]/repos/base-hw`.

Do you have any idea on how to work around this or even on how to improve the build system in order to support this use case?

Cheers
 Johannes

[1] https://github.com/genodelabs/genode/pull/1926