initconst adjustments

- add .init.rodata to INIT_DATA, and group all initconst flavors
  together
- move strings generated from __setup_param() into .init.rodata
- add .*init.rodata to modpost's sets of init sections
- make modpost warn about references between meminit and cpuinit
  as well as memexit and cpuexit sections (as CPU and memory
  hotplug are independently selectable features)

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>