kconfig: gconf: give a proper initial state to the Save button

Currently, the initial state of the "Save" button is always active.

If none of the CONFIG options are changed while loading the .config
file, the "Save" button should be greyed out.

This can be fixed by calling conf_read() after widget initialization.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>