faux-mgs update should take a hubris archive instead of a raw .bin image

[jgallagher]

@mkeeter asked about the format faux-mgs expects for SP updates. Currently it wants the raw final.bin created by hubris's xtask dist that is written directly to the SP's flash, but it would be nicer for Frankenrack bringup if it took a hubris archive instead. This work could start now and pull the .bin out of the archive; in the future it could also parse out other update pieces (FPGA images, etc.) once they're added to the archive.