Some SoCs allow to store multiple boot images on a device in order to
improve robustness. This adds a -r option to barebox_update to indicate
we do not want to make an update but instead repair/refresh an existing
image. Handlers which want to support this feature must set the
BBU_HANDLER_CAN_REFRESH flag during registration.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>