CI: Prepare split of test_kernels

What

Based on a comment from @ole.kruger in !11822 (merged), here is a preliminary step where we focus on moving everything RISC-V specific outside of kernels.mk.

The goal is for kernels.mk to do nothing more than make -C src/risc_v when interacting with the RISC-V project.

Why

test_kernels is long so we want to split it, but it turns out we can use this opportunity to reduce the project technical debt at the same time. It would be lazy not to use this opportunity.

How

Modify kernels.mk and src/risc_v/Makefile.

Manually testing the MR

If the CI is happy… to be honest, I can’t run the Tezt tests locally for now, still looking for the correct magic invocation.

One also needs to check that the same jobs are executed.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR