Bug 970658 - Cache downloaded extensions in build_stage

Instead of adding a root dependency, you should make `$(STAGE_DIR)/additional-extensions/downloaded.json` a dep of `extensions`.
Otherwise we get this error when running make extensions with empty profile:
```
$ DEBUG=1 make extensions
Makefile:135: NO_FTU_PING=1
cp -r tools/extensions/* profile-debug/extensions/
cp -r /home/alex/gaia/build_stage/additional-extensions/* profile-debug/extensions/
cp: cannot stat `/home/alex/gaia/build_stage/additional-extensions/*': No such file or directory
make: *** [extensions] Error 1
```