Ported Anaconda /etc/grub.conf symlink logic to boxgrinder-build

This patch brings the symlink from /etc/grub.conf to /boot/grub/grub.conf into boxgrinder for RHEL-based distributions. The original logic exists in the Anaconda project's source code, in pyanaconda/bootloader.py, beginning at line 1202.

This is necessary because the /sbin/new-kernel-pkg script depends on it. Without this simple symbolic link, RHEL machines created with boxgrinder will never rewrite the grub.conf file after updating a kernel. It will silently fail. This behavior has been observed on both CentOS and Scientific Linux, and likely applies to all RHEL-based distributions.

Tests are included.