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>

Makefile

@@ -234,6 +234,13 @@ modalai_voxl2: modalai_voxl2_slpi
 all_config_targets: $(ALL_CONFIG_TARGETS)
 all_default_targets: $(CONFIG_TARGETS_DEFAULT)
 
+# DEB package targets: builds _default config, then runs cpack.
+# Multi-processor boards (e.g. VOXL2) chain companion builds automatically
+# via existing cmake prerequisites.
+%_deb:
+	@$(call cmake-build,$(subst _deb,_default,$@)$(BUILD_DIR_SUFFIX))
+	@cd "$(SRC_DIR)/build/$(subst _deb,_default,$@)" && cpack -G DEB
+
 updateconfig:
 	@./Tools/kconfig/updateconfig.py