More serial console fixes for installer
This ticket continues the work started in the 2 related tickets. It will address:
1) Installer always enables serial as secondary console only, as result of which there is no installer menu on serial. Previously there was a menu item in GRUB for that.
2) Installer should set boot_serial="YES" option for the first boot depending on serial enabled in it, otherwise in UEFI mode part of output goes only to video console, and so may be nowhere.
3) After install we should also set boot_serial="YES" option in loader.conf reasonably.