loader.efi: detect console from ConOut/ConOutDev
Walk devices listed in ConOut (ConOutDev) variable(s) and set up console accordingly.
We need to check both variables because ConOut may be missing. In fact, both of them may be missing, in which case, we default to efi console (video). I have witnessed only laptop missing both ConOut and ConOutDev, therefore fall back to video should be justified (no serial on laptops).
Testing done: tested on my supermicro (physical system). For test, one should remove console and ttyX-mode lines from /boot/solaris/bootenv.rc and remove console line from /boot/defaults/loader.conf.