Adjust handling of package infos.

* add HAIKU_PACKAGING_ARCH, which is set to the target packaging 
  architecture
* introduce support for generic package infos, which are package infos
  that are the same for all architectures, except for the declaration
  of the package architecture itself
* move package info files underneath architecture-specific or generic
  folder
Add new package haiku_cross_devel_wrapper.

* this package wraps the haiku_cross_devel package (i.e. it contains
  that package in /develop/cross)
* the wrapper package is meant to be installed into the system 
  hierarchy, from where haikuporter will fetch the contained package
  when needed