mkimg previously, even if you specified -P 4k, would not align
MBR images to a 4k boundary. This hasn't been necessary for MBR
for almost 20 years.
As 4k drives are very common, change mkimg to default to a 4k
alignment. I would be fine w/ uping this to 16k or 64k.
This gives images the benefit w/o having to track down all the
various scripts that make an image to fix them.