W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://packages.ros.org/ros/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
W: Failed to fetch http://packages.ros.org/ros/ubuntu/dists/xenial/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654
解决方法：
$ apt-key adv --recv-keys --keyserver keyserver.ubuntu.com F42ED6FBAB17C654