Do you agree on the fact that app makefiles have to be run from the root one, with `APP=camera make`?
We now get some exception when we try to run make in an app directory (for camera and settings).
```
~/gaia/apps/camera$ make
rm -rf /shared
rm -rf /style
cp -rp ../../shared /
cp: cannot create directory `/shared': Permission denied
make: *** [/js/main.js] Error 1
```
May be we should add the following code to all still existing Makefiles:
```
ifndef XPCSHELLSDK
$(error This Makefile needs to be run by the root gaia makefile. Use `make APP=email` from the root gaia directory.)
endif
```
(taken from email's Makefile)

If you do agree with that, then everything would go through the root Makefile,
so that STAGE_APP_DIR env variable can always be set, by also setting it when calling build.js script right here:
  https://github.com/yurenju/gaia/blob/stage/Makefile#L158
Then, here, in xpcshell-common, we can assume env.get("STAGE_APP_DIR") will exist in case of app build modules.
(It only affect STAGE_APP_DIR. APP_DIR wouldn't change.)