kernel: use systemimage_intermediates on devices with no vendor

Change I8b6040e03589bde2af4e161c891ded237a9bb335 did not account for
devices without a separate vendor partition, causing build to fail with
the following:

FAILED: ninja: '/file_list.txt', needed by 'out/target/product/joan/obj/KERNEL_OBJ/arch/arm64/boot/Image.gz-dtb', missing and no known rule to make it

Use systemimage_intermediates instead on said devices.

Test: m kernel
Co-authored-by: Bruno Martins <bgcngm@gmail.com>
Change-Id: If9eb75a1bef98345e9c137c2463e392d50be239b