uboot: support -b option for mkenvimage

When generating the environment image the target endianess should
be taken in account for CRC calculation purposes. For big endian
targets the -b flag should be passed to the mkenvimage tool.

Signed-off-by: Tiago Brusamarello <tiago.brusamarello@datacom.ind.br>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>