stand/lua: Defer kernel/module loading until boot or menu escape

Loading the kernel and modules can be really slow. Loading before the menu
draws and every time one changes kernel/boot environment is even more
painful.

Defer loading until we either boot, auto-boot, or escape to loader prompt.
We still need to deal with configuration changes as the boot environment
changes, but this is generally much quicker.

This commit strips all ELF loading out of config.load/config.reload so that
these are purely for configuration. config.loadelf has been created to deal
with kernel/module loads. Unloading logic has been ripped out, as we won't
need to deal with it in the menu anymore.

Discussed in part with:	allanjude

stand/lua/core.lua

@@ -26,6 +26,8 @@
 -- $FreeBSD$
 --
 
+local config = require('config');
+
 local core = {};
 
 -- Commonly appearing constants
@@ -180,10 +182,12 @@ function core.setDefaults()
 end
 
 function core.autoboot()
+	config.loadelf();
 	loader.perform("autoboot");
 end
 
 function core.boot()
+	config.loadelf();
 	loader.perform("boot");
 end