OVA image creation is now fully integrated into build process

There are no complex external dependencies for OVA creation
anymore. An OVF file is directly maintained, and raw disks
are converted to streamOptimized VMDK using VBoxManage.

Added single loop device support to the chroot environment.

Separated boot/iso/ova phases into independent helper scripts,
invoked from "mkimage".

Removed unnecessary LC_ALL=C assignments at build-time.

Removed git snapshot creation script.

Clarified dependencies in Syslinux/GRUB configuration file
comments.