doc: Update Arduino tutorial
Add noflo-ui as submodule

We currently release a special version of noflo-ui together with
microflo, and to track this a submodule is very practical.
This arrangement will probably change in future when NoFlo UI has
releases of their own.
Update release target of Makefile

We now create a release which contains:
* MicroFlo Arduino library
* Our customized NoFlo UI (prebuilt)
* MicroFlo itself
This is much more convenient for end-users, and also avoids
need for maintaining a Makefile in our noflo-ui fork.
fixup! doc: Update Arduino tutorial
doc: More braindump