prexport.bbclass: avoid export for native and crosssdk

prexport.bbclass: avoid export for native and crosssdk

* The prservice is only used by do_package, the native or crosssdk
  doesn't have do_package.
* Change WARN to NOTE when no AUTOPR found, the prexport_handler()
  checks all the parsed recipes, but a lot of them may not be built, for
  example, the lower version or broken recipes. So change WARN to NOTE to
  not confuse user.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>