Add support for using the variables that m5 was compiled with for

Add support for using the variables that m5 was compiled with for
determining which parameters belong to a class.  This allows us to
remove the disable flag since it is not the correct model
for variable checking anyway.

objects/BaseCPU.mpy:
    Use the FULL_SYSTEM environment variable to enable or disable
    parameters.
sim/pyconfig/m5config.py:
    remove the disable flag since it is not the correct model
    for variable checking.

--HG--
extra : convert_revision : a8ccb78ba16d23006225df282a09187d32557608