Allow extra make args to ./build-qemu -- arg0 arg1

configure args are now ./build-qemu --extra-config-args '"aa a" bb'

Also factor out arg names with other builds through _add_argument.

Fix #113.

build-linux

@@ -75,13 +75,8 @@ so you might want to --clean the build first.
 Run `make modules_install` after `make`.
 '''
         )
-        self.add_argument(
-            'extra_make_args',
-            default=[],
-            metavar='extra-make-args',
-            nargs='*'
-        )
         self._add_argument('--force-rebuild')
+        self._add_argument('extra_make_args')
 
     def build(self):
         build_dir = self.get_build_dir()