github.py: Introduce cachable parameter
github.py: Introduce cachable parameter

A new parameter in the github provider section controls if a core
can be cached, or if it has to be refetched every time. Set this
to false for all cores that are tracking branches rather than
pointing to a single commit

lowRISC/fusesoc,lowRISC/fusesoc,olofk/fusesoc,olofk/fusesoc