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/python/gem5/runtime.py

@@ -42,6 +42,9 @@ def get_supported_isas() -> Set[ISA]:
     """
     supported_isas = set()
 
+    if not buildEnv["BUILD_ISA"]:
+        return {ISA.NULL}
+
     if "TARGET_ISA" in buildEnv.keys():
         supported_isas.add(get_isa_from_str(buildEnv["TARGET_ISA"]))