enable booti for imx8.
boot_addr_start for booti should be the addr of Image rather than
boot.img, so need read Image into hdr->kernel_addr.
change the offset for bootloader.
booti do not call android_image_get_kernel to init android env.
booti can't load boot.img, so it can't init android env.
init android env through android_image_get_kernel.

Change-Id: Ifb990ee9c5710ce7bd5fa9a0d4221dcb0e52d341
Signed-off-by: sanshan zhang <sanshan.zhang@nxp.com>