[oe] Variable override warning

[Perpelica Alexander]

Hi!
Can  I get some warnings about replacing variables which I set in 
local.conf.
Maybe bitbake must have such mode when he can write to console some info 
about what he did:
1. Variable is unset so I set to this value  (?=) in file ...
2. Variable is defined but I replace with new value (=) in file ...
3. Variable was changed with prefix(postfix) in file ...

For new users (for my :) ) is unclear where and who change and what change.