lualoader: Re-work menu skipping bits

This is motivated by a want to reduce heap usage if the menu is being
skipped. Currently, the menu module must be loaded regardless of whether
it's being skipped or not, which adds a cool ~50-100KB worth of memory
usage.

Move the menu skip logic out to core (and remove a debug print), then check
in loader.lua if we should be skipping the menu and avoid loading the menu
module entirely if so. This keeps our memory usage below ~115KB for a boot
with the menu stripped.

Also worth noting: with this change, we no longer explicitly invoke autoboot
if we're skipping the menu. Instead, we let the standard loader behavior
apply: try to autoboot if we need to, then drop to a loader prompt if not or
if the autoboot sequence is interrupted. The only thing we still handle
before dropping to the loader autoboot sequence is loadelf(), so that we can
still apply any of our kernel loading behavior.