nfsroot for RHEL 6

Dracut has subsumed all the initrd cleverness we had here.

All that is left is:
- minimal dracut module to strip kernel mods and fake
  init=/etc/rc.nfsroot root=dhcp kernel command line options
- rc scripts which try various methods to make root read-write and exec init
- tools for maintaining pxelinux.cfg and grubby hooks to keep it updated

kdump and mklivecd support is still pending.