distro: Imply USB_STORAGE when USB is available

When you support distro boot and you support USB, you usually want to
also support booting from USB storage.

Reflect that in the Kconfig, so that we don't have to explicitly add
USB storage support to every defconfig individually.
Reported-by: NAKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: NAlexander Graf <agraf@suse.de>

Kconfig

@@ -86,6 +86,7 @@ config DISTRO_DEFAULTS
 	select SUPPORT_RAW_INITRD
 	select SYS_LONGHELP
 	imply CMD_MII if NET
+	imply USB_STORAGE
 	imply USE_BOOTCOMMAND
 	help
 	  Select this to enable various options and commands which are suitable