When I try to implement the EDK design without any processor in PlanAhead tool, it outputs an error like the one below.
*** Running ngdbuild with args -intstyle ise -p xc6slx150tfgg676-3 -uc "system_stub.ucf" -bm "system_stub.bmm" "system_stub.edf"
ERROR:NgdBuild:634 - Cannot open input BMM file 'system_stub.bmm'
A BMM file is not needed for this design because there is no processor in the EDK design. It seems that this issue is caused by no processor in the EDK design because I do not see this error when the EDK design has a processor. The EDK design without a processor can also be implemented successfully in Project Navigator.
This issue has been resolved in the PlanAhead 14.2 and Vivado 2012.2 tools.