uboot: support -r option for mkenvimage

Some boards are configured in u-boot to store multiple redundant copies of
the environment image in flash.  For these boards, it is required to pass
the -r flag, when generating a boot environment image using mkenvimage.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>