scons: Update the Kconfig build options

The CL updates the Kconfig:
1. Replace the USE_NULL_ISA with BUILD_ISA
2. The USE_XXX_ISAs are depends on BUILD_ISA
3. If the BUILD_ISA is set, at least one of USE_XXX_ISAs must be set
4. Refactor the USE_KVM option

Change-Id: I2a600dea9fb671263b0191c46c5790ebbe91a7b8

src/cpu/o3/SConscript

@@ -30,7 +30,7 @@ import sys
 
 Import('*')
 
-if not env['CONF']['USE_NULL_ISA']:
+if env['CONF']['BUILD_ISA']:
     SimObject('FUPool.py', sim_objects=['FUPool'])
     SimObject('FuncUnitConfig.py', sim_objects=[])
     SimObject('BaseO3CPU.py', sim_objects=['BaseO3CPU'], enums=[