gem5 build system improved

Encapsulated gem5-specific code into wrapper functions to separate the
build process (Fail: CMake, gem5: scons). Added some gem5-related FIXMEs.

Another CMake related FIXME added. +some cosmetics.

Change-Id: Id84b480127b1f13aed6a0ee97f3583f410d531c5

cmake/gem5.cmake

@@ -3,5 +3,37 @@ if(BUILD_GEM5)
   message(STATUS "[${PROJECT_NAME}] Building gem5 variant ...")
   SET(VARIANT gem5)
 
-  #set(gem5_src_dir ${PROJECT_SOURCE_DIR}/simulators/gem5)
+  set(gem5_src_dir       ${PROJECT_SOURCE_DIR}/simulators/gem5)
+  set(gem5_wrapper       ${PROJECT_SOURCE_DIR}/src/core/sal/gem5)
+  set(gem5_build_config  build/ARM/gem5.debug)
+  set(core_count         9)
+
+  # FIXMEs:
+  # - incremental builds working?
+  # - dependency for modified .cc files not correctly checked
+  # - core_count should be derived from the parent make -jX parameter
+  # - make gem5_build_config configurable in CMake
+  #   (alternative: gem5_build_config is set based on the CMake build
+  #    config, e.g., "Debug" or "Release")
+
+  # Enable ExternalProject CMake module
+  include(ExternalProject)
+
+  # Use cmake's external project feature to build gem5 (and link FailGem5)
+  ExternalProject_Add(
+    FailGem5_binary_external # the (unique) name of this custom target (= external project)
+    # Disable update, patch and configure step:
+    UPDATE_COMMAND ""
+    PATCH_COMMAND ""
+    CONFIGURE_COMMAND ""
+    SOURCE_DIR ${gem5_src_dir}
+    BINARY_DIR ${gem5_src_dir}
+    # Build gem5 using scons build system:
+    BUILD_COMMAND scons EXTRAS=${gem5_wrapper} ${gem5_build_config} -j${core_count}
+    # Disable install step (for now)
+    INSTALL_COMMAND ""
+  )
+
+  # Build "fail" library first (will be statically linked to gem5)
+  add_dependencies(FailGem5_binary_external fail)
 endif(BUILD_GEM5)