For AVG you already can update definitions .
1) Boot AVG Rescue CD from USB
2) Select Update
3) Online update
4) Wait for the update to finish

or you can use offline update (i tried only online update and it works , later will check for offline and write again ) :
1. Create a new AVG updates folder in your Pen drive or hard drive. Copy and paste the downloaded update file on this newly created directory.
2. Boot from AVG Rescue CD and select Update, which shows the following options
Online : Update from the internet
Offline: Update from the directory.
Download ownload update files from the internet.
3. Now select offline option to navigate through partition and choose AVG updates folder. Run the update. After the update use “Return “ to come to scan. Then you scan your computer for latest virus threats.