Add support for OVN-K images in OKD builds for ARM architecture

[ggiguash]

As a follow-up of #84, we need to provide support for building OVN-K images in the OKD build script.

This code is currently commented out:

• microshift/src/okd/build_images.sh

  Line 256 in 4260fee

  # ovn_kubernetes_microshift_image

• microshift/src/okd/build_images.sh

  Lines 365 to 366 in 4260fee

  	# [ovn-kubernetes-base]="${TARGET_REGISTRY}/ovn-kubernetes-base:${OKD_VERSION}-${TARGET_ARCH}"
  	# [ovn-kubernetes-microshift]="${TARGET_REGISTRY}/ovn-kubernetes-microshift:${OKD_VERSION}-${TARGET_ARCH}"