Flatten_graph() called too often

[Risto97]

Currently flatten_graph() is called every time a property is read from the IP block library.

Simply it is the safest option, as it makes sure that none of the links changed since the last flatten_graph() call.
E.g. in case a new ip_link() call was done.

Although the performance of SoCMake is still good, flatten_graph() is still a big part of the runtime in the configuration stage.

An alternative to explore is that ip_link() can set a property like GRAPH_MODIFIED, which flatten_graph() would clear.
This way flatten_graph() will be done only when there are changes.