Luma modified to always autoboot to the configured DSi autoboot title ID regardless of autobooting configuration, to allow bootloaders to choose between regular boot or DSi boot instead of having to change boot options in Luma configuration every time.
Configure DSi homebrew autoboot as normal, leave autoboot to Off in Luma settings, then configure your bootloader to run the official Luma boot.firm when you want to boot into 3DS mode, or dsi.firm when you want to boot into DSi mode.