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

SConstruct

@@ -264,6 +264,7 @@ kconfig_actions = (
     'guiconfig',
     'listnewconfig',
     'menuconfig',
+    'savedefconfig',
     'setconfig',
 )
 
@@ -316,6 +317,17 @@ Kconfig tools:
         scons menuconfig build/foo/bar
 
 
+        savedefconfig:
+        Save a defconfig file which would give rise to the current config.
+        For instance, you could use menuconfig to set up a config how you want
+        it with the options you cared about, and then use savedefconfig to save
+        a minimal config file. These files would be suitable to use in the
+        defconfig directory. The second argument specifies the filename for
+        the new defconfig file.
+
+        scons savedefconfig build/foo/bar new_def_config
+
+
         setconfig:
         Set values in an existing config directory as specified on the command
         line. For example, to enable gem5's built in systemc kernel:
@@ -856,6 +868,12 @@ for variant_path in variant_paths:
         elif kconfig_action == 'menuconfig':
             kconfig.menuconfig(env, kconfig_file.abspath, config_file.abspath,
                     variant_path)
+        elif kconfig_action == 'savedefconfig':
+            if len(kconfig_args) != 1:
+                error('Usage: scons defconfig <build dir> <defconfig file>')
+            defconfig_path = makePathAbsolute(kconfig_args[0])
+            kconfig.savedefconfig(env, kconfig_file.abspath,
+                    config_file.abspath, defconfig_path)
         elif kconfig_action == 'setconfig':
             kconfig.setconfig(env, kconfig_file.abspath, config_file.abspath,
                     ARGUMENTS)