West Upgrade to automatically upgrade the Zephyr SDK

[robperkel]

What do you think about adding a command into west to automatically upgrade the major version of the SDK.

In the spirit of apt-get, maybe this command would be something like "west upgrade".

Behind the scenes, this command would perform the following actions:

1. (Make a backup?)
2. Update packages
3. Fetch new branches
4. Change branches (to the desired tag)
5. Pull the changes