kconfig: fix another bug of "make savedefconfig"

In some cases, the last lines of SPL or TPL are not output to a file.
The entries remaining in the "unmatched" variable must be flushed.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>