examples/filesystem: default to littlefs2
examples/filesystem: default to littlefs2

The upstream version of littlefs is what we call littlefs2.
'littlefs' is kept for legacy compatibility, but is not actively
developed upstream.

So make littlefs2 the default for the filesystem example.

jasonatran/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,miri64/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT