A warning when building with commits that are not in the mianifest

[rakons]

It happens me over and over, that I build a code forgetting the call to west update.
It would be really usefull if we would have a warning during build. It may work on cmake level.
Something like:

warning: Some repositories are not aligned with the current manifest. Try running 'west update' to get rid of this warning.

What do you think?