x86: zboot: Create a separate ZBOOT option for zboot logic

Most of the functionality of zboot is contained in the logic which
handles a zimage. Create a separate Kconfig for the logic so that it can
(later) be used without the command itself being enabled.

Enable ZBOOT by default on x86, with the command depending on that. The
existing 'imply' can therefore be removed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>