Rename misleading local variable
Rename misleading local variable

The local variable 'size' is actually the offset of the end, and not the
size.  Rename it to reflect this reality.

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

utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot