stand/lua: reload previously loaded kernel at config-load/reload

r329550 introduced config.kernel_loaded. config.load() doesn't provide a
means of overriding the kernel to load, but that likely isn't necessary as
it will not be a common case. When loading the kernel, just attempt to load
the kernel previously loaded and specified in config.kernel_loaded.

If we haven't loaded a kernel yet, config.kernel_loaded will be unset/nil
and the "default"/first kernel found will be loaded. If we've loaded a
kernel, we'll try to load that same kernel again and fallback to the default
kernel if we need to.

This in also in support of upcoming boot environment support.