fix(tflm): vendor third-party deps to enable reproducible builds

tflite-micro's Makefile fetches 5 third-party dependencies (flatbuffers,
kissfft, gemmlowp, ruy, pigweed, plus CMSIS/CMSIS-NN for ARM targets)
via wget and git clone at build time. This breaks reproducible builds:
the build output depends on GitHub and pigweed.googlesource.com being
reachable and serving the exact pinned archives, which is not guaranteed
and has broken in the past when GitHub silently regenerated archive zips.

Bump the tflite_micro submodule to the new px4/vendored-deps branch on
PX4/tflite-micro, which commits the deps directly under
tensorflow/lite/micro/tools/make/downloads/. Upstream's download scripts
already no-op with "already exists, skipping the download" when the
target directories are present, so no Makefile patches are needed.

Drop the add_custom_command, build_tflm_native target, and the empty
generate_cc_arrays.py shim from CMakeLists.txt. They existed solely to
drive the (now-removed) make third_party_downloads invocation and to
overwrite the submodule's generate_cc_arrays.py to keep that invocation
quiet. With the download step gone, none of that machinery is needed,
and the submodule stays clean after a build.

Verified builds against the new submodule:
- px4_sitl_neural (linux/amd64 devcontainer)
- mro_pixracerpro_neural (linux/amd64 devcontainer)
- px4_fmu-v6xrt_allyes (linux/amd64 and linux/arm64 devcontainers)

Fixes #27054

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

src/lib/tensorflow_lite_micro/CMakeLists.txt

@@ -72,25 +72,9 @@ if(CONFIG_LIB_TFLM)
 	${TFLITE_DOWNLOADS_DIR}/cmsis
 	)
 
-	set(TFLM_BUILD_TIMESTAMP ${CMAKE_CURRENT_BINARY_DIR}/tflm_build_complete.timestamp)
-	add_custom_command(
-		OUTPUT ${TFLM_BUILD_TIMESTAMP}
-		COMMAND ${CMAKE_COMMAND} -E copy_if_different
-			${CMAKE_CURRENT_SOURCE_DIR}/generate_cc_arrays.py
-			${CMAKE_CURRENT_SOURCE_DIR}/tflite_micro/tensorflow/lite/micro/tools/generate_cc_arrays.py
-		# TODO maybe change this if building for other architectures
-		COMMAND make -f tensorflow/lite/micro/tools/make/Makefile MICRO_LITE_EXAMPLE_TESTS= MICRO_LITE_BENCHMARKS= MICRO_LITE_TEST_SRCS= MICRO_LITE_INTEGRATION_TESTS= third_party_downloads
-		# Create timestamp file to mark completion
-		COMMAND ${CMAKE_COMMAND} -E touch ${TFLM_BUILD_TIMESTAMP}
-		WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/tflite_micro
-		COMMENT "Downloading TFLM third party dependencies"
-		DEPENDS git_tflite-micro
-		VERBATIM
-	)
-	add_custom_target(build_tflm_native
-		DEPENDS ${TFLM_BUILD_TIMESTAMP}
-	)
-
+	# Third-party dependencies are vendored in the PX4/tflite-micro fork under
+	# tensorflow/lite/micro/tools/make/downloads/ so no network fetch is needed
+	# at build time. See the px4/vendored-deps branch for details.
 	px4_add_library(tensorflow_lite_micro ${TFLITE_MICRO_SRCS})
 
 	if(CONFIG_BOARD_TOOLCHAIN STREQUAL "arm-none-eabi")
@@ -98,7 +82,7 @@ if(CONFIG_LIB_TFLM)
 	endif()
 
 	target_include_directories(tensorflow_lite_micro PUBLIC ${TFLM_INCLUDE_DIRS})
-	add_dependencies(tensorflow_lite_micro build_tflm_native)
+	add_dependencies(tensorflow_lite_micro git_tflite-micro)
 	target_compile_features(tensorflow_lite_micro PRIVATE cxx_std_17)
 
 	target_compile_options(tensorflow_lite_micro PUBLIC