Set KERNEL_IMAGE_FORMAT to a bit of make which will determine the
format based on architecture. When referenced in the makefile,
make will evaluate it and get the correct image format string.
Set the snap version number back to 5.4.0-31.35 as the makefile
expects the version to be the same as the kernel pcakcage version,
not the linux-uc20-efi version.
* Switch to use EFI kernel image.
* Introduce a new make parameter KERNEL_IMAGE_FORMAT.
* We don't need debootstrap anymore. Instead we're now using gpg,
and depmod from the kmod package.