Feature #8093: loader.efi: cleanup loader main and implement comconsole
loader: add asprintf for libstand
The loader libstand is missing asprintf, so lets add it, as it would simplify our life. The implementation is actually quite simple - we let kvprintf() to count the size of the resulting string, we allocate the memory and then fill it.
As the current kvprintf does assume non-NULL buffer, the small update is needed there to make sure we wont dereference the NULL pointer.