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

Ideally, we wouldn't filter in `options.rebuildAppDirs.forEach(function(appDir) {`,
`options.rebuildAppDirs` would contain the final list of app to build, even when we specific APP=foo.
````
options.rebuildAppDirs = options.GAIA_APPDIRS.split(' ').filter(app => {
  let appDirFile = utils.getFile(appDir);
  return appRegExp.test(appDirFile.leafName);
});
```