generate eve sbom in build process

[deitch]

• adds a target make sbom, which takes the generated rootfs.tar, expands it to a temporary directory (excluding special devices, which would cause it to fail), runs syft on it, saves the output into the installer/ directory, and removes the tmpdir
• adds sbom to the dependencies of make eve, so that the sbom is generated with every build and included in the lfedge/eve docker image
• adds a command sbom, so that you can run docker run lfedge/eve sbom and get an sbom in spdx json format

[eriknordmark]

LGTM

[deitch]

@eriknordmark now that this is in, we will have an SBoM in every build. We can iterate on it by improving the content. For example, the Dockerfile ADD scanner, and the scanner you wrote, and kernel improvements, etc.