Respect EXECUTABLE_OUTPUT_PATH for bin/hex targets

The CMake variable in question is listed here:

http://www.cmake.org/Wiki/CMake_Useful_Variables#Locations

The current behaviour is when you set EXECUTABLE_OUTPUT_PATH is that the binary generated from add_executable ends up in the specified path, and then the bin/hex generation fails since it looks in the wrong spot.

The proposed logic still doesn't account for targets whose filenames differ from the target names— for example, by setting the OUTPUT_NAME property, but it's still a needed improvement.