make video init a runtime option, instead of a build time option. configurable in nvram. e.g. video disable/enable, type of init e.g. corebootfb/txtmode, etc

i've seen the code in coreboot handling these cases. right now, it's done per build time configuration

a single user on libreboot irc has been asking me to do headless roms for t60, but that simply won't do. having this as a runtime option makes more sense, to turn off the video init, but make "on" the default so that pulling the cr2032 re-enables video in case the user messes up

#85 make video init a runtime option, instead of a build time option. configurable in nvram. e.g. video disable/enable, type of init e.g. corebootfb/txtmode, etc