include/configs: apalis-imx8/verdin-imx8mm: rename kernel image variable

include/configs: apalis-imx8/verdin-imx8mm: rename kernel image variable

Variable "kernel_image" is used in boot.scr script only, that sets its
own default value to the constant string @@KERNEL_IMAGETYPE@@ in case
"kernel_image" is not set.
The default name of the kernel image shipped with BSP 5.x is "Image.gz".
Setting kernel_image="Image" as a pre-defined u-boot variable
breaks booting systems with modern versions of boot.scr, whereas
renaming it fixes booting with modern scripts and does not break working
of earlier versions of boot.scr.

While at it also update the copyright period, rather than hard-coding
fdtfile default fdt_board to dev for the Verdin iMX8M Mini and fix its
closing #endif comment.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>