vboot_reference: Support MTD devices in dump_kernel_config This CL implements a read function that works with MTD devices in dump_kernel_config. BUG=chromium:457862 BRANCH=none TEST=make runtests TEST=try on storm_nand Change-Id: Id784d422de64e7918b163005c0b426d727d2115e Reviewed-on: https://chromium-review.googlesource.com/249271 Reviewed-by: Nam Nguyen <[email protected]> Commit-Queue: Nam Nguyen <[email protected]> Trybot-Ready: Nam Nguyen <[email protected]> Tested-by: Nam Nguyen <[email protected]>