Add-on helper: add foreign buildsystem support

[notspiff]

Allow taking advantage of foreign buildsystems. Make the CMake target copy a binary built by the external project. This is used for libretro-based add-ons where the add-on binary is not specific to Kodi.

This has been sitting in retroplayer for a few years. I have extended it with support for a custom data directory. It's split so @garbear doesn't have to deal with a conflict.

[fetzerch]

+1 for the change, verified couple of times that it works with libretro cores.
Just since you anyways touch those lines: mind changing the commands to lowercase? (if/else/install, ...)

[hudokkow]

Don't bother changing to lowercase. I have a branch with that done.
Although not perfect, this helps. Rest is grep and s/old/new/

[fetzerch]

Ok, otherwise good to go?

jenkins build this with addons please

[hudokkow]

👍
Build is fine. The usual suspects fail on Linux and peripheral.joystick didn't liked the includes change but I'm waiting on garbear's input.

Needs a rebase.

[notspiff]

rebased.

[hudokkow]

Thanks!
jenkins build this with addons please

[hudokkow]

Thanks!