libblkid: add Flash-Friendly File System (f2fs) support

This adds support for detecting Flash-Friendly File System (f2fs) to libblkid.
Based on work by Sven-Göran Bergh at http://www.mail-archive.com/busybox@busybox.net/msg17874.html

Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>
tests: add f2fs image test

Signed-off-by: Alejandro Martinez Ruiz <alex@nowcomputing.com>