Add reactor uc

[LasseRosenow]

Initial draft (scripts still don't work)

make all in root should work after lfcg was run

[erlingrj]

This looks reasonable to me!

[LasseRosenow]

I have removed the scripts for now and greatly simplified the Makefile, by moving things over to riot.mk in reactor-uc see: lf-lang/reactor-uc#166

I noticed a chicken-egg issue though.
Right now the LFC copies reactor-uc into the src-gen folder.
But without reactor-uc already available somewhere else we cannot use the LFC in the first place, as it is part of reactor-uc.

That is why for now I depend on the REACTOR_UC_PATH variable to be set to build this example.
Inside riot.mk we can then get the LFC path based on the REACTOR_UC_PATH variable.

What about we include reactor-uc just as a top-level submodule? maybe together with RIOT in a deps folder, or just on root.
This way this all would work out of the box and REACTOR_UC_PATH would be very clear as well.

[erlingrj]

@LasseRosenow, will you tag me for a review again after making modifications as discussed in today's meeting?

[LasseRosenow]

@erlingrj Ready for review again

[erlingrj]

This looks great. I have given you write access to the repo so you can push PRs directly to this instead of a fork