loader.efi: Default to serial if we don't have a ConOut variable

In the EFI implementation in U-Boot no ConOut efi variable is created,
this cause loader to fallback to TERM_EMU implementation which is very
very very slow (and uses the ConOut device in the system table anyway).
The UEFI spec aren't clear as if this variable needs to exists or not.

Reviewed by: imp, kevans