fs/iso9660: add isolinux support

After all the preparation commits, this commit finally adds the
iso9660 support itself. Besides adding a new Config.in entry, a little
bit of .mk code and the isolinux.cfg default configuration, not much
is needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>