Initial support for the ESP32 C6 board (backport #274)

[mergify]

This make it possible to set board target and build for the RISC-V board on the ESP32 C6 Board.

The C6 is not added to the CI workflow in this PR.

---

This is an automatic backport of pull request #274 done by Mergify.

[mergify]

Cherry-pick of c5825cc has failed:

On branch mergify/bp/humble/pr-274
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit c5825cc.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/ci.yml
	modified:   README.md
	modified:   esp32_toolchain.cmake.in
	modified:   libmicroros.mk

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/nightly.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally