The UI is optional and need to be explicitly activated, since I
don't want existing users to suddenly have an information-spewing
thing show up on their network.
iPXE appears to have *really* poor performance (orders of magnitude worse) if
it doesn't know the length of the kernel/initrds that it's downloading. Without
this change, booting CoreOS takes longer than I've had patience to wait. With
this change, the bottleneck becomes the network transfer speed.
Fixes#10.
