sim: Match structure definition better
sim: Match structure definition better

This padding field is defined as a u8 and a u16.  Clarify the marshaling
code to match instead of just looping over bytes.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot