efi_loader: support CTRL+\ - CTRL+_

In the extended text input protocol support input of control letters
0x1c - 0x1f.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>