Make CPU_MODELS a sticky build option.

Make CPU_MODELS a sticky build option.
This causes a crash if you're using scons 0.96.1 *and* you specify
more than one CPU model.  Since the .isa scanner now works with 0.96.91
then upgrading should not be an issue.  For now we're only using one CPU
model (SimpleCPU) so there isn't even a pressing need to upgrade yet.

build/SConstruct:
    Make CPU_MODELS a sticky option.
    This causes a crash if you're using scons 0.96.1 *and* you specify
    more than one CPU model.  Since the .isa scanner now works with 0.96.91
    then upgrading should not be an issue.  For now we're only using one CPU
    model (SimpleCPU) so there isn't even a pressing need to upgrade yet.

--HG--
extra : convert_revision : d8319c4cd5c937c2c033270cef850d19b805d256