fs/iso9660: introduce new variables

In preparation to the introduction of the support for other
bootloaders that Grub, this commit introduces two new variables:

 - ROOTFS_ISO9660_BOOTLOADER_CONFIG_PATH, which gives the full path to
   the bootloader configuration file

 - ROOTFS_ISO9660_BOOT_IMAGE, which gives the relative path of the
   main bootloader image, as needed by genisoimage's -b option.

There are no functional changes made, as the variables are for now
always set to the same value.

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