efi_loader: Add console interface

One of the basic EFI interfaces is the console interface. Using it an EFI
application can interface with the user. This patch implements an EFI console
interface using getc() and putc().

Today, we only implement text based consoles. We also convert the EFI Unicode
characters to UTF-8 on the fly, hoping that everyone managed to jump on the
train by now.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>