fastboot: simplify the Kconfig logic

Currently, the fastboot item in menuconfig is a comment followed by a
boolean option withan empty prompt, followed by a menu:

        *** FASTBOOT ***
    [*]
          Fastboot support  --->

This is not "nice-looking" at all...

Change the logic to make the boolean option a "menuconfig" rather than a
mere "config", so that all dependent options gets groupped under a menu.
The layout is now:

        *** FASTBOOT ***
    [*] Fastboot support  --->

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>