system.img file is missing after building Android source
It looks like the build did not complete successfully. Run make again. It should try to resume from the point the build failed, whereupon it'll encounter the same error again. Probably the error message will tell you what went wrong: possibly it's a problem with the software you need to build the source, or likely the way you've "modified" the build has caused the problem.

If you can't work out yourself what the error message means, then our sister site Stack Overflow will be able to offer specific help if you tell them the message and your situation.