image: add MBIST-enabled Cosmo images for mfg

[luqmana]

Updated helios-build with some changes that rely on oxidecomputer/amd-host-image-builder#255 & oxidecomputer/amd-host-image-builder#256 to allow for generating an image from a patch. As well as allowing for gating image generation on a feature flag (-F).

With that, this introduces a pair of MBIST-enabled ROM images for Cosmo (cosmo-mbist & cosmo-mbist-noaggr) to be generated alongside the usual cosmo.rom (oxidecomputer/hardware-cosmo#33) (gated behind feature mbist). The -noaggr variant should run ~5min quicker. Both images pull in oxidecomputer/dmar-report to be available at /usr/bin/dmar-report. Once MBIST has completed and the OS has booted it can be used to collect the margining results in tarball to be further processed & analyzed (e.g. with dmar_tar2csv.py).

[jclulow]

I think it might have been better to use the JSON syntax aware mk_rom_config()-style approach for this, rather than textual patch files -- but we need to get moving so we'll use it as-is for now. Thanks!