Update submodule v2.0 to latest Fri Apr 13 20:26:31 CDT 2018

- v2.0 in PX4/Firmware (23017d051b7ff6686c8c8ff025a8e98af094780a): https://github.com/mavlink/c_library_v2/commit/163d4c84b9eb58a02cab94d039f54c303977d991
    - v2.0 current upstream: https://github.com/mavlink/c_library_v2/commit/c55dd0ec312a667f28b0cab4f1fe697d2888c00c
    - Changes: https://github.com/mavlink/c_library_v2/compare/163d4c84b9eb58a02cab94d039f54c303977d991...c55dd0ec312a667f28b0cab4f1fe697d2888c00c

    c55dd0e 2018-04-12 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/292874beed6bf756be45421bd2ae2420de921f2b
e2916e5 2018-04-10 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/012765b923546224ef928caeb86c72d5a78a433a