efi_loader: rework event handling for console

Preread the next key in the console timer event.

The EFI_SIMPLE_TEXT_INPUT_EX_PROTOCOL requires to trigger registered key
notification functions based on the prefetched key.

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