build-whitelist: do not add new options to whitelist when update

If somebody adds references to new CONFIG options in source files,
they will be added in the whitelist when we sync it.  (For example,
if we run scripts/build-whitelist.sh against commit 42f75050,
new options CONFIG_SPL_DFU_SUPPORT and CONFIG_USB_XHCI_UNIPHIER will
appear in the list.)

In order to make steady progress of Kconfig migration, we want to
only decrease whitelist options, but never increase.

So, when we update the whitelist, we should create a temporary list,
then take the intersection of the temporary one and the current one.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>