linux-initrd: Store Linux modules in a normal store directory.

linux-initrd: Store Linux modules in a normal store directory.

* gnu/system/linux-initrd.scm (expression->initrd): Remove #:linux and
  #:linux-modules parameters.  Remove call to
  'float-linux-module-directory'.
  (base-initrd): Add call to 'float-linux-module-directory'.  Use it in
  #:linux-modules argument in the gexp.  Remove #:linux and
  #:linux-modules arguments to 'expression->initrd'.
* gnu/build/linux-initrd.scm (build-initrd): Remove
  #:linux-module-directory parameter.  Don't create 'modules'
  sub-directory.
* gnu/build/linux-boot.scm (boot-system): Mentin that LINUX-MODULES is a
  list of absolute file names.  Don't prepend "/modules/" to
  LINUX-MODULES.
* doc/guix.texi (Initial RAM Disk): Adjust accordingly.