loader.efi: There are systems without ConOut, also use ConOutDev

Conout does contian the default output device name.
ConOutDev does contain all possible output device names, so we can
use it as fallback, when there is no ConOut.

PR: 253253

(cherry picked from commit 2bd4ff2d8911009283e4e615ca4aad35a845f48b)