board/freescale/common/imx/imx8-bootloader-prepare.sh: fix FIT external position

When using imx-mkimage 6.1.36_2.1.0 or later, an additional data structure
is inserted in the generated image. The FIT external data position passed
to the uboot mkimage program needs to be adjusted accordingly.

Fixes: https://bugs.busybox.net/show_bug.cgi?id=15973
Fixes: 72de7890

 ("package/imx-mkimage: bump version to lf-6.1.36-2.1.0")
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
[Sébastien: Tested on i.MX8MM EVK and i.MX8MP EVK]
[Sébastien:
 - fix subject
 - add Tested-by tag
 - fix Fixes tags
]
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>