lualoader: Only loadelf before boot/autoboot if no kernel loaded

Back when I "fixed" the loading of kernel/modules to be deferred until
booting, I inadvertently broke the ability to manually load a set of kernels
and modules in case of something bad having happened. lualoader would
instead happily load whatever is specified in loader.conf(5) and go about
the boot, leading to a panic loop as you try to rediscover a way to stop the
panicky efirt module from loading and fail miserably.

Reported by: me, sadly