loader: fall back to term_emu on efi console with serial backend

In case of efi console having serial backend (video + serial or only serial),
we need to stick with old emulator till we can draw console.

Eventually we would need to get console terminal emulator to be removed
from serial console because the serial link already has the terminal.

However, we need to implement comconsole on all efi platforms first, then
we need the ability to draw console, so we do not have to use SimpleTextOutput
protocol (which will write both on video and serial in case of multiplexed
ComOut).

Differential Revision: https://reviews.freebsd.org/D22161