Add a `debug` configuration option for FlexPRET

[magnmaeh]

It would be very helpful for debug and testing purposes to be able to access parts of the processor not available otherwise. E.g., like being able to overwrite the internal time register. In this case, one could write a test that

overwrite_time(0xffffff00);
fp_delay_until(0xffffffff);

This way, one would not need to simulate the entire time from time 0 to time 0xffffffff to check if the test works as expected.