core: turn kernel_init.c and panic.c into submodules of core

[kaspar030]

Contribution description

This PR changes the default kernel_init to be a submodule called "core_init".
That makes it possible to completely bypass it, as done by riotboot.
Previously, riotboot depended on a specific link order to properly override kernel_init()
core_init is made a default module, riotboot simply disables it.

Same with panic.c turned into the core_panic module.

Testing procedure

• basic compilation and execution should still work
• riotboot should be tested
• actual code sizes should be unchanged

CI can do that.

Issues/PRs references

#13238

[nmeum]

What about core_panic, shouldn't it be disabled as well?

[kaspar030]

What about core_panic, shouldn't it be disabled as well?

Yes, that'll be another PR.

[kaspar030]

Yes, that'll be another PR.

actually, that doesn't make sense. They both require exactly the same testing.

[kaspar030]

• rebased, changed panic.c similar to kernel_init.c

@nmeum Does this fix the issues with linktime re-ordering?

[nmeum]

@nmeum Does this fix the issues with linktime re-ordering?

Yes, it does seem to fix it, did a few tests locally. I also just pushed your two commits on the #13176 link time reordering branch. Since I can just rebase my branch against master as soon as this PR is merged. If you want, you can wait to see if it passes on the CI.

[nmeum]

Tested with link time reordering (see #13176), seems to solve the problem at hand.

[haukepetersen]

Change makes a lot of sense! Tested with native and arduino-due, everything still behaves as expected.

ACK.

[kaspar030]

Thanks @nmeum @haukepetersen!