feat(boards/modalai/voxl2): add Debian packaging framework

Add a scalable .deb packaging framework for VOXL2, built on the
existing cmake/package.cmake CPack infrastructure. The framework
handles multi-processor boards by having the POSIX (_default) build
own the .deb and pull in the companion SLPI build's artifacts.

Board-specific files:
- cmake/package.cmake: CPack variable overrides (name, deps, version)
- cmake/install.cmake: install() rules for all .deb contents
- debian/postinst: px4-* symlinks, DSP signature, directory setup
- debian/prerm: service stop, symlink cleanup
- debian/voxl-px4.service: systemd unit (after sscrpcd)

Infrastructure changes:
- cmake/package.cmake: hook for board-specific CPack overrides
- platforms/posix/CMakeLists.txt: hook for board install.cmake
- Makefile: %_deb pattern rule (build _default, then cpack -G DEB)
- CI: auto-discover _deb targets, collect .deb artifacts, upload
  to GitHub Releases

Future boards: add cmake/package.cmake + cmake/install.cmake and
CI discovers it automatically. No new file formats or tools needed.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/build_all_targets.yml

@@ -265,5 +265,7 @@ jobs:
         with:
           draft: true
           prerelease: ${{ steps.upload-location.outputs.is_prerelease == 'true' }}
-          files: artifacts/*.px4
+          files: |
+            artifacts/*.px4
+            artifacts/*.deb
           name: ${{ steps.upload-location.outputs.uploadlocation }}