Improve build-all.ps1 script

[jordimontana82]

In order to making the cloning and building of all the different repos and packages a bit more seamless, the build script could be improved to:

• [ ] Show a warning if you're not in a DynamicsValue parent folder: to highlight the fact repos should have the same parent folder
• [ ] Prompt any warnings if some of the target frameworks or SDKs are missing
• [ ] Check if the local-packages folder exist, otherwise create it, via confirm
• [ ] Check if a folder for each repo exists, otherwise git clone it, via confirm
• [ ] Run git-pull.ps1 on all repos after they've been downloaded
• [ ] Build & publish everything into local-packages