Adds ability to have out-of-tree uORB message definitions

[ksschwabe]

If the EXTERNAL_MODULES_LOCATION variable has been set, and the
EXTERNAL_MODULES_LOCATION/msg/ directory exists containing a
CMakeLists.txt file with the following format:

    set(config_msg_list_external
      message1.msg
      message2.msg
      message3.msg
      ...
      PARENT_SCOPE
      )

then the messages defined in config_msg_list_external are added to the
msg_files list in Firmware/msg/CMakeLists.txt and are used to generate uORB
message headers. The generated uORB message headers are placed in the same
location as the normal uORB message headers in the build directory, namely,
<build_dir>/uORB/topics. The uORB topic sources are generated in
<build_dir>/msg/topics_sources.

This should help users who do a lot of out-of-tree development. This will keep the custom msg definitions out of the PX4/Firmware tree.

[hamishwillee]

@ksschwabe When this is integrated, do you think you could review whether we need to update the documentation correspondingly? Specifically, two documents may be impacted:

• https://dev.px4.io/en/advanced/out_of_tree_modules.html
• https://dev.px4.io/en/middleware/uorb.html

[ksschwabe]

@hamishwillee: https://dev.px4.io/en/middleware/uorb.html can remain unchanged, but the https://dev.px4.io/en/advanced/out_of_tree_modules.html will have to be updated. I will generate a PR for that if/when this is integrated.

[bkueng]

looks good to me