Bug 969215 - Execute |make| should only build specific apps we modified

Now that we have `rebuildAppDirs` we should ensure it contains the final set of apps we are going to rebuild.
So it shouldn't be options.GAIA_APPDIRS.split(' '); when we pass APP=foo.
It should be that:
```
options.rebuildAppDirs = options.GAIA_APPDIRS.split(' ').filter(app => {
  let appDirFile = utils.getFile(appDir);
  return appRegExp.test(appDirFile.leafName);
});
```