Poc kernel dedup with symlink from overlay2

[ycardaillac]

Use a symlink between boot and the overlay

https://balena.fibery.io/default/My-current-cycle-1819#Work/Project/Deduplicate-kernel-on-balenaOS-rootfs-2300

Contributor checklist

• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
• Change-type present on at least one commit
• Signed-off-by is present
• The PR complies with the Open Embedded Commit Patch Message Guidelines

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