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>

cmd/fastboot/Kconfig

 comment "FASTBOOT"
 
-config FASTBOOT
-	bool ""
+menuconfig FASTBOOT
+	bool "Fastboot support"
 
-menu "Fastboot support"
-	depends on FASTBOOT
+if FASTBOOT
 
 config USB_FUNCTION_FASTBOOT
 	bool "Enable USB fastboot gadget"
@@ -89,4 +88,4 @@ config FASTBOOT_MBR_NAME
 
 endif # USB_FUNCTION_FASTBOOT
 
-endmenu
+endif # FASTBOOT