mavlink:Use CONFIG_BOARD_ROOT_PATH instead of string constant

src/modules/mavlink/mavlink_ftp.cpp

@@ -1160,7 +1160,7 @@ bool MavlinkFTP::_validatePathIsWritable(const char *path)
 	// Don't allow writes to system paths as they are in RAM
 	// Ideally we'd canonicalize the path (with 'realpath'), but it might not exist, so realpath() would fail.
 	// The next simpler thing is to check there's no reference to a parent dir.
-	if (strncmp(path, "/fs/microsd/", 12) != 0 || strstr(path, "/../") != nullptr) {
+	if (strncmp(path, CONFIG_BOARD_ROOT_PATH "/", 12) != 0 || strstr(path, "/../") != nullptr) {
 		PX4_ERR("Disallowing write to %s", path);
 		return false;
 	}