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> Signed-off-by: Ross Burton <ross.burton@intel.com>
