Temporary fix to unresponsive HSS serial interface

This is an initial fix to the debug serial port being unresponsive once Linux has booted. The fix uses a temporary HSS repository while the full fix is published on the Microchip HSS repository. Furthermore, the use of this temporary HSS repository should remain until the gateware scripts allow using a specific hash of the HSS to allow locking the gateware to a specific official HSS version.