fs/iso9660: put the Buildroot boot entry first in Grub menu.lst

The ISO9660-specific Grub menu.lst contains two entries: one entry to
chainload the bootloader available in the first hard drive, and
another entry to boot the Buildroot system.

However, it defaults to booting the first entry, i.e chainloading
what's on the first hard drive. For a Buildroot generated system, this
is quite odd: we're not even booting the system built by Buildroot.

So, switch the two entries, and put the Buildroot boot entry first.

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