soc: arm: nxp_imx: update iMX RT boot header to handle CONFIG_XIP=n

Update iMX RT boot header to support CONFIG_XIP=n, when the image is not
linked into the flash space. Also, update the boot header to correctly
calculate the flash used size within the boot data header. This field is
used by the iMX boot ROM to determine how much data to copy when running
from RAM, so using the correct size fixes an issue where the ROM would
copy more data than needed.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>