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-buildroot

@@ -61,16 +61,10 @@ Don't add our overlay which contains all files we build without going through Bu
 This prevents us from overwriting certain Buildroot files. Remember however that you must
 still rebuild the Buildroot package that provides those files to actually put the Buildroot
 files on the root filesystem.
-'''
-        )
-        self.add_argument(
-            'extra-make-args', default=[], nargs='*',
-            help='''\
-Extra arguments to be passed to the Buildroot make,
-usually extra Buildroot targets.
 '''
         )
         self._add_argument('--force-rebuild')
+        self._add_argument('extra_make_args')
 
     def build(self):
         build_dir = self.get_build_dir()