AP_HAL_PX4: added main program as include

libraries/AP_HAL_PX4/AP_HAL_PX4_main.h

@@ -0,0 +1,10 @@
+// this is included at the end of every sketch
+extern "C" __EXPORT int SKETCH_MAIN(int argc, char *argv[]);
+int SKETCH_MAIN(int argc, char *argv[]) 
+{
+    printf("%s starting\n", SKETCHNAME);
+    hal.init(NULL);
+    setup();
+    for(;;) loop();
+    return OK;
+}