tools: moveconfig: support CONFIG_SYS_EXTRA_OPTIONS cleaning

We mostly move config options from board header files to Kconfig,
but sometimes config defines come from CONFIG_SYS_EXTRA_OPTIONS.

Historically, CONFIG_SYS_EXTRA_OPTIONS originates in boards.cfg,
which was used as a central database of configuration prior to the
Kconfig conversion.

Now, we want to migrate to primary entries in Kconfig rather than
option list in CONFIG_SYS_EXTRA_OPTIONS, so it should be helpful to
have the tool to cleanup CONFIG_SYS_EXTRA_OPTIONS automatically.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>