Use ninja files by default in the Flow API

[olofk]

The flow API abstracts the chaining of tools to the EdaCommands class which is responsible for creating a final makefile.

Due to make being a bit of a pain on certain platforms it is probably just as well to generate a ninja file instead. If Edalize then also adds a dependency on ninja, it would be one less moving part.

I would like to see the EdaCommands class being subclassed to use different flowgraph execution methods, e.g. ninja, make, plain bat/sh files or directly run them from Python

[olofk]

https://github.com/olofk/edalize/issues/293 seems to be yet another reason to get rid of makefiles