Enable Edalize flow API support

[olofk]

This enables support for the new flow API in Edalize, which will eventually supersede the current Edalize API. For now these two APIs live side by side. FuseSoC users who wish to remain on the old Edalize API doesn't need to do anything. For users wanting to use the flow API, set the flow key in a target to one of the supported flows, and set the flow_options to control flow-specific options. These two keys have precedence over the default_tool and tools keys currently used in the target section.

This is also one of the two major changes I want to get in before the FuseSoC 2.0 release, so please take a look and comment accordingly

[olofk]

Ping @imphil

[olofk]

You're right. Documentation is very much needed. Will start writing.

[olofk]

I have also encountered some unforseen issues that will require an API rewrite. Specifically, changing the flow graph using flow options (e.g. adding a frontend) doesn't work very well because it's not possible to set any tool options for tools that aren't in the default flow graph. Will do an attempt to solve this with a two-stage approach where we first evaluate all flow options, then ask Edalize to provide the tool options that are relevant for that flow configuration

[olofk]

Alright then. I have added some basic documentation. @imphil, could you take another look?

[olofk]

I went ahead and merged this