lualoader: unload upon kernel change if a kernel was previously loaded

In the majority of cases, a kernel is not loaded before we hit the menu.
However, if a password is set, we'll trigger autoboot and have loadelf'd
beforehand. We also need to take into account one dropping to the loader
prompt and twiddling with things manually; if they try to toggle through
kernels, we'll assume they mean it.

Reported by:	trasz
MFC after:	 3 days

stand/lua/menu.lua

@@ -312,6 +312,9 @@ menu.welcome = {
 				    #all_choices .. ")"
 			end,
 			func = function(_, choice, _)
+				if loader.getenv("kernelname") ~= nil then
+					loader.perform("unload")
+				end
 				config.selectKernel(choice)
 			end,
 			alias = {"k", "K"},