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>

platforms/posix/CMakeLists.txt

@@ -80,7 +80,8 @@ target_link_libraries(px4 PRIVATE uORB)
 # install
 #
 
-# TODO: extend to snapdragon
+# Generic install rules (skipped when board provides its own install.cmake)
+if(NOT EXISTS "${PX4_BOARD_DIR}/cmake/install.cmake")
 
 # px4 dirs
 install(
@@ -94,6 +95,8 @@ install(
 	USE_SOURCE_PERMISSIONS
 )
 
+endif() # NOT board install.cmake
+
 # Module Symlinks
 px4_posix_generate_symlinks(
 	MODULE_LIST ${module_libraries}
@@ -112,6 +115,11 @@ if(EXISTS "${PX4_BOARD_DIR}/cmake/upload.cmake")
 	include(${PX4_BOARD_DIR}/cmake/upload.cmake)
 endif()
 
+# board defined install rules for .deb packaging
+if(EXISTS "${PX4_BOARD_DIR}/cmake/install.cmake")
+	include(${PX4_BOARD_DIR}/cmake/install.cmake)
+endif()
+
 # board defined link libraries
 if(EXISTS "${PX4_BOARD_DIR}/cmake/link_libraries.cmake")
 	include(${PX4_BOARD_DIR}/cmake/link_libraries.cmake)