fs/iso9660: use 'depends on' instead of 'select' for bootloader

In preparation to the introduction of the support for other
bootloaders than Grub, this commit switches from having the iso9660
support "select" the necessary bootloader to using a "depends on".

The main motivation is that the isolinux bootloader support will
really need to do a "depends on", due to the need of having a certain
option enabled, but this option being a kconfig "choice".

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