fs/iso9660: add support for grub2

After having extended the iso9660 support to allow isolinux as an
alternative to grub, this commit adds grub2 as a third
alternative. With the previous work done to support isolinux, adding
support for grub2 is fairly trivial.

[Thomas: set timeout to 10 seconds and not 5 seconds, in order to
match the configuration used for grub.]

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