gnu: Lower initrd makers from packages to monadic procedures.

* gnu/packages/linux-initrd.scm: Remove.
* gnu/system/linux-initrd.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Adjust accordingly.
* gnu/system.scm (<operating-system>): Change default 'initrd' value
  to (gnu-system-initrd).
  (operating-system-derivation): Bind 'operating-system-initrd'.  Pass
  'menu-entry' an initrd file name instead of a package.
* gnu/system/grub.scm (grub-configuration-file): Expect 'initrd' to be
  file name.