U-Boot can be chainloaded from vendor firmware on ARM64 chromebooks from
a GPT partition (roughly the same as in doc/chromium/chainload.rst), but
an appropriate image header must be built-in to the U-Boot binary by
enabling LINUX_KERNEL_IMAGE_HEADER.

This header has a field for an image load offset from 2MiB alignment
which must also be customized through LNX_KRNL_IMG_TEXT_OFFSET_BASE.
Set it equal to SYS_TEXT_BASE by default for Rockchip boards, which
happens to make this offset zero and works fine on chromebook_kevin
both for chainloading and bare-metal use.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>