MA-18913 Load dtb according to kernel address

MA-18913 Load dtb according to kernel address

As the kernel image will get bigger after enabling some debug
tools, 64MB kernel max size is not enough. Load the dtb to the
address right after linux kernel instead of setting fixed offset
to the start of kernel image.

The "kernel_size" in boot image header is the physical size of
"Image" instead of the memory size which the linux requires to
boot.

Test: boots on imx8mp/imx8qm.

Change-Id: I5946c8530610ff0742f2a911aaeccf7636938b94
Signed-off-by: Ji Luo <ji.luo@nxp.com>
Reviewed-by: Wang Haoran <elven.wang@nxp.com>
(cherry picked from commit c1f83790077516a64397467fce5880be0871f615)