bootm: Move arm64-image processing later

If the image is compressed, then the existing check fails, since the
header is wrong.

Move the check later in the boot process, after the kernel is
decompressed. This allows use of bootm with compressed kernels, while
still permitting an uncompressed kernel to be used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>