Add functionality to scripts for subsequent builds

[ianw1974]

Currently the scripts rely on /home/git/zimbra being empty, so in their current state they will potentially fail if you attempt to re-run 02-build-zimbra.sh multiple times.

The main issues here being:

• zm-build will already exist and thus will not do anything - therefore we will be missing any potential changes for future releases (easy to fix).
• all 59+ directories that the build process downloaded in /home/git/zimbra will exist - therefore need to add to the scripts a cleanup option to remove all these existing directories before building a new release (also easy to fix).

these will be added to the scripts when I make the next builds for future 9.x releases as well as modified config.build to automatically build for the latest version. I will be adding this functionality and testing locally first before I publish to ensure that on this github repo, there is always a completely working version.

To circumvent the problem in the meantime, you can manually delete the contents of /home/git/zimbra and then re-run 02-build-zimbra.sh

Now /home/git/zimbra is empty and ready for re-building. This will be added to future releases so will automatically take care of this for you.

[ianw1974]

The scripts have been combined into one single script now with functions. This will make things easier, and save having to run two different scripts.

The function above for deleting the old build contents has been added, so if a build has been attempted, it will delete the contents so that a new build can be made.

These new changes will be commited shortly as am currently testing them to ensure that they work as intended.