cubian-nandinstall enhancement

Pull request is for install.sh only.

Added choice of 2 or 3 partitions.
Increased nanda size to 10MB.
Set nandb size to 50MB if 3 partitions chosen.
/boot placed on nandb if 3 partitions chosen.
/boot symlink placed on nandc if 3 partitions chosen.
/etc/fstab configured by number of partitions chosen.
exclude.txt requires no modifications.
u-boot requires no modifications.