build: envkernel: fix building out-of-tree

We used to use .output as the default kbuild_out but this was removed recently, add it back and rewrite the Makefile to fix building again.

changed title from build: envkernel: default to .output again to build: envkernel: fix building out-of-tree

added 3 commits

• 32045775 - 1 commit from branch master
• 932ac187 - build: envkernel: default to .output again (MR 2504)
• aa847501 - build: envkernel: overmount outdir/Makefile to use local include (MR 2504)

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Pipeline skipped: all already passed except for 1 integration job, which failed due to known CI runner issues, unrelated to the MR at hand.

merged