Ref project: https://code.google.com/p/autoddvpn/wiki/jffs
Add a shell to check VPN status. Only support jffs.
Step 1: Copy the shell to /jffs/pptp/
Step 2: Chmod +x checkvpn.sh
Step 3: Change startup command from '/jffs/pptp/run.sh' to '/jffs/pptp/checkvpn.sh'