efi_loader: support Unicode text input

Up to now the EFI_TEXT_INPUT_PROTOCOL only supported ASCII characters.
With the patch it can consume UTF-8 from the console.

Currently only the serial console and the console can deliver UTF-8.
Local consoles are restricted to ASCII.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>