Add native sboms with buildkit

We need to have SBoMs on our container images, in order to consume and compose them at a later stage for all of eve. This PR on eve does that for all of the in-eve-repo packages.

For this one, we need to do 2 things:

1. Do our custom scanner/adder, which places a custom SBoM in the container image (already done earlier)
2. Run the buildkit scanner with SBoM generation

For 2, it is straightforward. Taking one sample branch Makefile.eve, we already build with docker:

kernel-build-%: sbom Makefile.eve
	@echo "Building kernel version $(BRANCH):$(VERSION)-$* with compiler $*"
	docker buildx build \
	--build-arg="SOURCE_DATE_EPOCH=$(SOURCE_DATE_EPOCH)" \
	--build-arg="KBUILD_BUILD_TIMESTAMP=$(KBUILD_BUILD_TIMESTAMP)" \
	--build-arg="LOCALVERSION=$(VERSION)$(DIRTY)" \
	--platform $(PLATFORM) -t lfedge/eve-kernel:$(BRANCH)-$(VERSION)$(DIRTY)-$* --load -f Dockerfile.$* .

For this to work. we need to ensure we are running at least buildkit v0.11 (actual GA, not an -rc version), and add the flag --sbom=true to the image build.

That should do it. Once it is in place, you can check that it is there, see the blog post

Start with the Makefile.eve kernel-build-% rule and its docker buildx build command, then check how the repository selects or documents its BuildKit version. Done means the build uses BuildKit v0.11 or newer, enables SBOM generation, and the resulting image contains an SBOM that can be verified as described in the issue.