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: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>