efi_loader: multi part device paths to text

Our current implementation of
EFI_DEVICE_PATH_TO_TEXT_PROTOCOL.ConvertDevicePathToText() truncates multi
part device paths after the first part. We should convert all parts.

Render device path instance ends as commas. This is not explicitly
described in the UEFI spec but mimics what EDK II does.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>