test: bootdev: Do not require USB to compile test

This test will block compilation of the entire test suite
on platforms without USB support. Make the extern
"usb_started" conditional on USB host or gadget and
define a dummy flag if neither is enabled.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>