loader: For EFI, if we don't have ConOut, try ConIn

Try ConIn if we don't have a ConOut variable. ConIn will contain HID
devices and/or serial devices. We currently just search for serial
devices and will use them instead of video with the current code. While
ConIn w/o ConOut is fairly common on laptops, is kinda rare on servers.
Some refinement may be needed in the future if servers come to
light. This is also minimal to allow possible integration into 13.3
release.

MFC After: 1 week
Sponsored by: Netflix
Reviewed by: dab, tsoome
Differential Revision: https://reviews.freebsd.org/D43714