linux-hikey960: remove boot and dts images build
linux-hikey960: remove boot and dts images build

We switched to UEFI and GRUB:
* dts image isn't needed anymore
* another boot image is now built from edk2-hikey960 recipe, which
  contains GRUB EFI application only.

Signed-off-by: Fathi Boudra <7cd6e55cc07a66bc71d7901a4cddfe6c22aab659@linaro.org>

kraj/meta-96boards,mrchapp/meta-96boards,96boards/meta-96boards,fboudra/meta-96boards,kraj/meta-96boards