update-u-boot: support gnu sed

reported by @saba in chat, if gnu sed is used, update-u-boot fails to find the boot partition. basically, it doesn't support using \d: https://stackoverflow.com/questions/14671293/why-doesnt-d-work-in-regular-expressions-in-sed/14671365#14671365

Thanks @Arnavion for the help figuring this out!