An option to only update src tree when running make update from src/.

I use the make.conf options for specifying my src, docs and ports
supfiles. One thing that I feel would be useful is to have an option to
specify that a user only wants their src tree to be updated when running
from src. I feel this could be useful, as I run make update from my
ports dir when I want my ports tree updated.

Or is there an option already that I am just missing?

Thanks for your time,

-- 
Adam Gregoire <ebola_at_psychoholics.org>