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/kvm/Kconfig

@@ -30,5 +30,4 @@ config KVM_ISA
 config USE_KVM
     depends on KVM_ISA != ""
     bool "Enable hardware virtualized (KVM) CPU models"
-    default y if KVM_ISA != ""
-    default n
+    default y

src/cpu/minor/SConscript

@@ -40,7 +40,7 @@
 
 Import('*')
 
-if not env['CONF']['USE_NULL_ISA']:
+if env['CONF']['BUILD_ISA']:
     SimObject('BaseMinorCPU.py', sim_objects=[
         'MinorOpClass', 'MinorOpClassSet', 'MinorFUTiming', 'MinorFU',
         'MinorFUPool', 'BaseMinorCPU'],

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=[

src/cpu/o3/probe/SConscript

@@ -37,7 +37,7 @@
 
 Import('*')
 
-if not env['CONF']['USE_NULL_ISA']:
+if env['CONF']['BUILD_ISA']:
     SimObject('SimpleTrace.py', sim_objects=['SimpleTrace'])
     Source('simple_trace.cc')
     DebugFlag('SimpleTrace')

src/cpu/simple/SConscript

@@ -28,7 +28,7 @@
 
 Import('*')
 
-if not env['CONF']['USE_NULL_ISA']:
+if env['CONF']['BUILD_ISA']:
     SimObject('BaseAtomicSimpleCPU.py', sim_objects=['BaseAtomicSimpleCPU'])
     Source('atomic.cc')
 

src/cpu/simple/probes/SConscript

@@ -28,6 +28,6 @@
 
 Import('*')
 
-if not env['CONF']['USE_NULL_ISA']:
+if env['CONF']['BUILD_ISA']:
     SimObject('SimPoint.py', sim_objects=['SimPoint'])
     Source('simpoint.cc')