Fix package installation in make-all-linux-project

There was a bug where missing packages would cause the script
to exit.

This change also makes several improvements:
- script can now be run from anywhere (not only the cocos root)
- apt-get is called only once to install all missing packages
  (this makes output much more informative on first use)
- added comment/docs to the top of the script
Merge pull request #2124 from sbc100/fix_make_all

fixed #1819: Fix package installation in make-all-linux-project.