fs/iso9660: introduce bootloader choice

In preparation to the introduction for the support of other
bootloaders than Grub, this commit adds a "choice ... endchoice" block
with just the Grub option, and adds some conditionals in the
iso9660.mk code for the Grub specific parts.

Of course, for now those conditionals are a bit useless with just this
commit, but they become useful with the followup commits.

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