scons: Hook up the savedefconfig kconfig helper.

This helper utility lets you save the defconfig which would give rise to
a given config. For instance, you could use menuconfig to set up a
config how you want it with the options you cared about configured, and
then use savedefconfig to save a defconfig of that somewhere to the
side, in the gem5 defconfig directory, etc. Then later, you could use
that defconfig to set up a new build directory with that same config,
even if the kconfig options have changed a little bit since then.

A saved defconfig like that can also be a good way to visually see what
options have been set to something interesting, and an easier way to
pass a config to someone else to use, to put in bug reports, etc.

Change-Id: Ifd344278638c59b48c261b36058832034c009c78

site_scons/gem5_scons/kconfig.py

@@ -33,6 +33,7 @@ _kconfig_helpers = {
     "GUICONFIG_PY": "guiconfig.py",
     "LISTNEWCONFIG_PY": "listnewconfig.py",
     "MENUCONFIG_PY": "menuconfig.py",
+    "SAVEDEFCONFIG_PY": "savedefconfig.py",
     "SETCONFIG_PY": "setconfig.py",
 }
 
@@ -130,6 +131,22 @@ def menuconfig(
         error("Failed to run menuconfig")
 
 
+def savedefconfig(env, base_kconfig, config_in, config_out):
+    """
+    Interface of handling savedefconfig.py of Kconfiglib
+    """
+    kconfig_env = _prep_env(env, base_kconfig, config_in)
+    kconfig_env["CONFIG_OUT"] = config_out
+    if (
+        kconfig_env.Execute(
+            '"${SAVEDEFCONFIG_PY}" '
+            '--kconfig "${BASE_KCONFIG}" --out "${CONFIG_OUT}"'
+        )
+        != 0
+    ):
+        error("Failed to run savedefconfig")
+
+
 def setconfig(env, base_kconfig, config_path, assignments):
     """
     Interface of handling setconfig.py of Kconfiglib