initrd: Rework build and install stages

initrd: Rework build and install stages

* Rewrite the Makefile to provide only the 'all' and 'install' targets.
  Do not separate the handling of the rescue tools and initrd image.
  Thus greatly simplify the whole logic.

  The 'all' target will build the rescue tools and initrd contents, and
  does not require root priviledge, while the 'install' target will
  install the rescue tools to the system, create the initrd image and
  install it.

* Update the top-level makefiles to call the new 'all' and 'install'.
  The 'buildworld' target now doesn't require root priviledge, same as
  before the refactoring in 9b724c0dcb2b9548a82d28d97e34375f64668669.

* Only provide the 'initrd' top-level target to keep it simple.  This
  target will install the rescue tools and initrd image built by the
  'buildworld', overwriting the existing ones.

* Clean up the nrelease/Makefile a bit, since 'installworld' will also
  install the rescue tools and initrd image in such a case.

* Update the mkinitrd.sh script a bit.

* Update the build(7) man page and clean up a bit.

Reviewed-by: swildner