Define default CONFIG_PREBOOT with right config option

The 44758771 commit removes CONFIG_PREBOOT but actually sets the USE_PREBOOT
Kconfig option which isn't CONFIG_PREBOOT and is also a bool option which means
we regress because 'usb start' isn't run when expected, it should also be run
for devices that have USB storage because keyboards aren't the only thing we
might need the USB bus for.

Fixes: 44758771

 ("arm: move CONFIG_PREBOOT="usb start" to KConfig")
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Cc: Jonas Smedegaard <dr@jones.dk>
Cc: Neil Armstrong <narmstrong@baylibre.com>

common/Kconfig

@@ -403,7 +403,6 @@ config BOOTCOMMAND
 
 config USE_PREBOOT
 	bool "Enable preboot"
-	default "usb start" if USB_KEYBOARD
 	help
 	  When this option is enabled, the existence of the environment
 	  variable "preboot" will be checked immediately before starting the
@@ -417,6 +416,7 @@ config USE_PREBOOT
 config PREBOOT
 	string "preboot default value"
 	depends on USE_PREBOOT && !USE_DEFAULT_ENV_FILE
+	default "usb start" if USB_KEYBOARD || USB_STORAGE
 	default ""
 	help
 	  This is the default of "preboot" environment variable.