coremanager.py: Expand user names correctly
fusesoc.in: Warn instead of exit on missing cores_root
fusesoc.in: Add init command

The init command checks out the orpsoc-cores repo and writes a
default fusesoc.conf file if it doesn't exist
fusesoc.in: Add update command

The update command iterates through all detected cores_root, and
if it detects an upstream orpsoc-cores repo attempts to pull it