bootm: don't call booti_setup for EFI images

On the arm64 architecture booti_setup() is called for EFI FIT images.
This function call fails because EFI images typically do not have a
kernel signature.

Check that the operating system property "os" of the image is "linux"
before invoking booti_setup().

Fixes: 487b5fa6 ("bootm: Handle kernel_noload on arm64")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>