build: split CMakeLists

- Put add_custom_command into seperate ChiselBuild module. Otherwise
  final target cannot correctly depends on scala sources.

npc/csrc/CMakeLists.txt

@@ -0,0 +1 @@
+add_subdirectory(${TOPMODULE})

npc/csrc/Flow/CMakeLists.txt

@@ -0,0 +1,11 @@
+include(ChiselBuild)
+add_executable(V${TOPMODULE} config.cpp main.cpp)
+
+verilate(V${TOPMODULE} TRACE COVERAGE THREADS
+  TOP_MODULE ${TOPMODULE}
+  PREFIX V${TOPMODULE}
+  SOURCES ${CHISEL_OUTPUT_TOPMODULE} ${CHISEL_OUTPUT_VERILATOR_CONF}
+  INCLUDE_DIRS ${CHISEL_OUTPUT_DIR}
+  VERILATOR_ARGS
+    "--vpi"   # Enable VPI
+)