Build Dracut-based ramdisks

This is an alternative to the ramdisk element that uses Dracut to provide the base system functionality instead of Busybox.

For elements that need additional drivers in the ramdisk image, a dracut-drivers.d feature is included that works in a similar fashion to the binary-deps.d feature. The element needing to add drivers should create a dracut-drivers.d directory and populate it with a single file listing all of the kernel modules it needs added to the ramdisk. Comments are not supported in this file. Note that these modules must be installed in the chroot first.

By default, the virtio, virtio_net, and virtio_blk modules are included so that ramdisks are able to function properly in a virtualized environment.