reconsider the /machine-action file

[hallyn]

Depending on what the boot is doing, we might want to provision, pre-install, just 'mount' (for simple livecd), or initrd-setup (for regular installed boot). Right now we require a hint in the mounted rfs to tell us what to do. That's not pretty. What other ways are there that we can know what to do?

A kernel cmdline 'machine.action=[install|boot|livecd] seems like a good option, but then we'd have to update stubby to whitelist that.