Improved bootstrap script for Linux

I added libsodium to bootstrap script for Linux because:

1. The install guide of toxcore uses a specific version of libsodium
2. Some distributions may not provide a prepacked libsodium or it might be the wrong version

Furthermore I added the option (-t or --tox) to only install libtoxcore. This can be used to simply update libtoxcore to the current head. Since the Windows prepacked zip archive already includes all dependencies I had to only modify the Linux version.