fs/iso9660: prepare cleaner kernel/initrd path handling

fs/iso9660: prepare cleaner kernel/initrd path handling

The iso9660.mk modifies the menu.lst file from Grub to set the correct
initrd/kernel image locations. However, with the upcoming support of
other bootloaders for iso9660 filesystems, we need to modify a bit
this logic.

Instead of relying on the specific details of the grub menu.lst
syntax, we introduce the __KERNEL_PATH__ and __INITRD_PATH__ magic
keywords, which iso9660.mk will replace by the appropriate
values. They can therefore be used where needed in grub menu.lst, and
in similar configuration files of other bootloaders, as will be
supported in the following commits.

Also, in order to be consistent with the soon to be introduced support
for having the root filesystem itself as iso9660, this commit changes
the installation location of the initrd and kernel. Instead of being
/initrd and /kernel, they become /boot/initrd and
/boot/$(LINUX_IMAGE_NAME).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>