Update submodule mavlink v2.0 to latest Sat Aug 4 01:26:42 UTC 2018 (#10143)

- v2.0 in PX4/Firmware (ca3e4a53): https://github.com/mavlink/c_library_v2/commit/2c4b0e9ff26ce4900a72335912e2604a1e9948de
    - v2.0 current upstream: https://github.com/mavlink/c_library_v2/commit/6404047979cadac62fecd75070cd995f1ca66ca2
    - Changes: https://github.com/mavlink/c_library_v2/compare/2c4b0e9ff26ce4900a72335912e2604a1e9948de...6404047979cadac62fecd75070cd995f1ca66ca2

6404047 2018-08-03 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/b4871443354d607298b6f67bedd6c473c69c659a
d0fb5b8 2018-08-03 PX4BuildBot - autogenerated headers for rev https://github.com/mavlink/mavlink/tree/1ba233efffb54a7ed634160cd11855b3f5021b74

v2.0 @ 64040479

-Subproject commit 2c4b0e9ff26ce4900a72335912e2604a1e9948de
+Subproject commit 6404047979cadac62fecd75070cd995f1ca66ca2