file-systems: Add support for F2FS.

* gnu/build/file-systems.scm (%f2fs-endianness): New syntax.
(f2fs-superblock?, read-f2fs-superblock, f2fs-superblock-uuid)
(f2fs-superblock-volume-name, check-f2fs-file-system): New procedures.
(%partition-label-readers, %partition-uuid-readers, check-file-system): Register them.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>