oniro-image: use x-gpt-efi-disk for SECO B68

This, coupled with the earlier EFI_PROVIDER change allows the generated
disk image to correctly boot with GRUB under QEMU/EFI. There is some
loss of functionality: the early-boot microcode patch system is gone,
for the moment, until we figure out how to bring it back with improved
and unified GRUB configuration files.

This was tested with:

        qemu-system-x86_64 -cpu IvyBridge -machine q35 -bios ovmf.qcow2 \
                oniro-image-base-seco-intel-b68.wic -m 1024 -nographic

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>