spike/config.cmake: Use variable that is defined

At the stage where this is processed, Kernel32 hasn't been defined yet.

src/plat/spike/config.cmake

@@ -17,7 +17,7 @@ cmake_minimum_required(VERSION 3.7.2)
 if(KernelPlatformSpike)
     config_set(KernelPlatform PLAT "spike")
     config_set(KernelPlatformFirstHartID FIRST_HART_ID 0)
-    if(Kernel32)
+    if(KernelSel4ArchRiscV32)
         list(APPEND KernelDTSList "tools/dts/spike32.dts")
     else()
         list(APPEND KernelDTSList "tools/dts/spike.dts")