Fix yarp_add_idl use when cross-compiling

[traversaro]

In YARP are two CMake macros to run yarpidl programs, yarp_idl_to_dir and yarp_add_idl.

When cross-compiling (for exmaple for Android or iOS) , it is not possible for yarp_idl_to_dir and yarp_add_idl to call the yarpidl_rosmsg and yarpidl_thrift provided by the YARP that is used for linking the program, as that version tipically does not work for the system on which we are cross-compiling. For this reason, it is important that there is a way for users to specify manually the yarpidl_rosmsg and yarpidl_thrift executables that yarp_idl_to_dir and yarp_add_idl use.

Even before this PR, yarp_idl_to_dir works fine with cross-compilaton by specifing the CMake cache variables YARPIDL_thrift_LOCATION and YARPIDL_rosmsg_LOCATION. If this variables are not set, the one provided by the YARP used to link the applications are used instead.

Before this PR, yarp_add_idl was not using the executable specified in the YARPIDL_thrift_LOCATION and YARPIDL_rosmsg_LOCATION variable, while after this PR also yarp_add_idl works fine in cross-compilation.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update the release notes by adding a file in doc/release/<target_branch>, based on your changes.

[randaz81]

Marked as draft as discussed with @traversaro