fs: iso9660: change boot menu entry text

Replace Grub-specific "menu.lst" with "menu config" in the
BR2_TARGET_ROOTFS_ISO9660_BOOT_MENU Kconfig entry text, and mention
missing grub.cfg for Grub 2.

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