initramfs: New modular initramfs for booting kernel from various media.

initramfs: New modular initramfs for booting kernel from various media.
* Design principles:
* This initramfs just mounts roots and runs init on it, nothing else.
* Whenever possible, existing kernel parameters are used. When extended
  boot methods are provided, new parameters mimick/adhere to existing
  conventions.
* System is modular/pluggable - each mount method is in own module with
  more or less well defined interface, it's possible to construct complete
  system with any combination of modules.
* Modules includes: initramfs-module-loop, initramfs-module-nfs, to boot
  from loop devices (including recursive loops) and NFS, correspondingly.