That way we avoid recompiling source files that don't depend on vehicles.
Change notes:
- UTILITY_SOURCE_EXTS: moved to ap_library
- Task generators indexes: ap_library handles that now and in a better way
- Use of AP_STLIB_FEATURES: it doesn't make sense anymore, since the stlib
taskgen has empty source list
- Flags and defines: passed down to ap_library through AP_LIBRARIES_OBJECTS_KW
- Set use='mavlink' in AP_LIBRARIES_OBJECTS_KW instead of calls to
bld.ap_stlib().
Currently we need to build a different binary for each type of copter frame.
This is a preparation for setting all those binaries to a group called
"copter". We create the groups for the rest of the main products for
consistency.
The task generator for the mavlink headers is given a name 'mavlink' and is
added as a dependency to all local static libraries, because all of them use
sources that include mavlink headers. It would be nice to have a good way to
detect sources that use mavlink headers or to declare list of libraries that
use the headers, so that the dependency could be added automatically.
Additionally, there's a minor change in syntax for the task generator: using
strings directly instead of lists.