Installation slides can now be localised

English slides for e.g. Raspbian stay in output/slides/Raspbian/
but localised slides can be placed in a language-code subdirectory.
For example you could put some French-localised slide images in
output/slides/Raspbian/fr/ and then NOOBS will display these
automatically during OS-installation if the NOOBS GUI has been set
to French. If there are no language-specific slides for the chosen
OS and language, then it'll simply display the English slides.
Language can be pre-set in recovery.cmdline

This allows you to set the default language that the NOOBS GUI will
be displayed in by editing a file on the SD card using your Windows
PC, before NOOBS has even booted for the first time.
Simply add a lang=xx option to cmdline.recovery (keeping everything
all on one line) where xx is a two-letter language code, e.g. add
lang=fr to display NOOBS in French, or lang=de for German, etc.
The language *can* still be changed within the GUI, this merely
determines which language will be selected first. If the entered
lanuage code isn't available, it'll obviously revert to English.
Make the command-line language-comparison case-insensitive

So that lang=DE in recovery.cmdline is treated the same as lang=de
Update output submodule HEAD used in superproject.
Merge branch 'localisable-installation-slides' of https://github.com/lurch/noobs into dev