Allow postponing of Bitbake Calls in Settings Dialog

[cordlandwehr]

I had/have the following use case: for a slightly more complicated Yocto setup (Bitbake shall be run in a container, inside the container a git credentials cache needs to be initialized) I did not get the Command Wrapper directly right, as it needs to be set inside plugin settings. While trying to retyping it, I got constant warnings and failures from not working Bitbake calls (well, I was still typing and not even ready to give it a next try ;) )

So I think it would make sense to have automatic execution of BitBake calls disabled until after the first successful run and before that, to have an explicit "test config" button.