A feed is a collection of packages and the metadata scan scripts expect Makefiles to reside two levels deep at least. Assuming that /home/onur/Desktop/OpenWrt_SDK/mypackages is the feed directory you setup in feeds.conf, then your OpenWrt package Makefile should be placed in /home/onur/Desktop/OpenWrt_SDK/mypackages/helloworld/Makefile. For each further package, add a new subdirectory with a new Makefile.
The feed directory itself should contain no Makefiles but only subdirectories with Makefiles representing packages.
Also remove the export TOPDIR overwrite. The appropriate environment is exported by scripts/feeds already