backend: add connection URL as arg to backend_bin

This allows backend_bin to accept the connection URL as an argument, as
well as -h or --help to show the usage.

backend/src/dronecode_sdk_server.cpp

@@ -1,6 +1,42 @@
 #include "backend_api.h"
+#include <iostream>
+#include <string>
+
+static void usage();
+
+static auto constexpr default_connection = "udp://:14540";
 
 int main(int argc, char **argv)
 {
-    runBackend("udp://14540", nullptr, nullptr);
+    if (argc > 2) {
+        usage();
+        return 1;
+    }
+
+    if (argc == 2) {
+        const std::string positional_arg = argv[1];
+        if (positional_arg.compare("-h") == 0 || positional_arg.compare("--help") == 0) {
+            usage();
+            return 0;
+        }
+        runBackend(positional_arg.c_str(), nullptr, nullptr);
+    }
+
+    runBackend(default_connection, nullptr, nullptr);
+}
+
+void usage()
+{
+    std::cout << "Usage: backend_bin [-h | --help]" << std::endl
+              << "       backend_bin [Connection URL]" << std::endl
+              << std::endl
+              << "Connection URL format should be:" << std::endl
+              << "  Serial: serial:///path/to/serial/dev[:baudrate]" << std::endl
+              << "  UDP:    udp://[bind_host][:bind_port]" << std::endl
+              << "  TCP:    tcp://[server_host][:server_port]" << std::endl
+              << std::endl
+              << "For example to connect to SITL use: udp://:14540" << std::endl
+              << std::endl
+              << "Options:" << std::endl
+              << "  -h | --help : show this help" << std::endl;
 }