build: Introduce ainfo, aerror functions

envsetup.sh calls make directly to set some build vars.  Any "bare" info or
error messages interfere with the logic, so they must be guarded with
checks for CALLED_FROM_SETUP.  This is tedious and error prone, so provide
an alternative that handles this logic automagically.

Change-Id: I9f9a355a6ae58f2b173cffce796e37be1e68238c