kconfig: fix 'Save As' menu of xconfig

The 'Save As' menu of xconfig is not working; it always saves the
kernel configuration into the default file irrespective of the file
chosen in the dialog box.

The 'Save' menu always writes into the default file, but it would
make more sense to write into the file previously chosen by 'Load'
or 'Save As'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>