Kconfig: Add EXPERT option

For similar reasons to why the Linux Kernel has an EXPERT option, we too
want an option to allow for tweaking of some options that while normally
should remain hidden, may need to be changed in some cases.

Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>