uorb: allow for more than 255 uORB messages

Increased size for ORB_ID from uint8_t to uint16_t

Created a type: orb_id_size_t = uint16_t.
There are still a couple of places where the size
of the ORB_ID is assumed to be less than 16-bits.
The places that I have found are commented regarding
this and can be found with a search on orb_id_size_t.

platforms/common/uORB/uORB.h

@@ -42,16 +42,17 @@
 #include <stdint.h>
 #include <stdbool.h>
 
+typedef uint16_t orb_id_size_t;
 
 /**
  * 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 */
+	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) */
+	orb_id_size_t  o_id;                /**< ORB_ID enum */
 };
 
 typedef const struct orb_metadata *orb_id_t;

platforms/common/uORB/uORBDeviceMaster.cpp

@@ -150,7 +150,7 @@ int uORB::DeviceMaster::advertise(const struct orb_metadata *meta, bool is_adver
 
 			// add to the node map.
 			_node_list.add(node);
-			_node_exists[node->get_instance()].set((uint8_t)node->id(), true);
+			_node_exists[node->get_instance()].set((orb_id_size_t)node->id(), true);
 		}
 
 		group_tries++;

platforms/common/uORB/uORBDeviceMaster.hpp

@@ -98,7 +98,7 @@ public:
 			return false;
 		}
 
-		return _node_exists[instance][(uint8_t)id];
+		return _node_exists[instance][(orb_id_size_t)id];
 	}
 
 	/**