initial minimal PX4_ROS2 platform and px4_ros2_default build (#20689)

- new ROS2 platform in PX4 intended for creating configs that build and run entirely in ROS2
 - PX4_CONFIG defaults to px4_ros2_default if no config specified and in a colcon workspace with ROS_VERSION=2
 - currently doesn't do much other than allow you to build px4 msgs interface package

platforms/ros2/include/uORB/uORB.h

@@ -0,0 +1,99 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2021 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+#pragma once
+
+
+#include <sys/types.h>
+#include <stdint.h>
+#include <stdbool.h>
+
+
+/**
+ * Object metadata.
+ */
+struct orb_metadata {
+	const char *o_name;		/**< unique object name */
+	const uint16_t o_size;		/**< object size */
+	const uint16_t o_size_no_padding;	/**< object size w/o padding at the end (for logger) */
+	const char *o_fields;		/**< semicolon separated list of fields (with type) */
+	uint8_t o_id;			/**< ORB_ID enum */
+};
+
+typedef const struct orb_metadata *orb_id_t;
+
+/**
+ * Maximum number of multi topic instances. This must be <= 10 (because it's the last char of the node path)
+ */
+#if defined(CONSTRAINED_MEMORY)
+# define ORB_MULTI_MAX_INSTANCES 4
+#else
+# define ORB_MULTI_MAX_INSTANCES 10
+#endif
+
+
+
+#define ORB_ID(_name)		&__orb_##_name
+
+#if defined(__cplusplus)
+# define ORB_DECLARE(_name)		extern "C" const struct orb_metadata __orb_##_name __EXPORT
+#else
+# define ORB_DECLARE(_name)		extern const struct orb_metadata __orb_##_name __EXPORT
+#endif
+
+
+#define ORB_DEFINE(_name, _struct, _size_no_padding, _fields, _orb_id_enum)		\
+	const struct orb_metadata __orb_##_name = {	\
+		#_name,					\
+		sizeof(_struct),		\
+		_size_no_padding,			\
+		_fields,				\
+		_orb_id_enum				\
+	}; struct hack
+
+/**
+ * Print a topic to console. Do not call this directly, use print_message() instead.
+ * @param meta orb topic metadata
+ * @param data expected to be aligned to the largest member
+ */
+void orb_print_message_internal(const struct orb_metadata *meta, const void *data, bool print_topic_name);
+
+#ifndef PX4_INFO_RAW
+# define PX4_INFO_RAW		printf
+#endif
+
+#ifdef __cplusplus
+
+#include <uORBTopics.hpp>
+
+#endif // __cplusplus