Prevent Kati from seeing MAKEFLAGS

We never want Kati to see MAKEFLAGS, as forcefully overriding variables
is terrible. The variables in MAKEFLAGS are still available in the
environment, so if part of the build wants input from the user, it
should be explicitly checking for an environment variable or using ?=

This commit shouldn't actually change any behavior -- the version of
Kati checked into our tree doesn't actually read MAKEFLAGS (the latest
version does, which is why this change is necessary). We're also already
clearing MAKEFLAGS during makeparallel, so the USE_GOMA case already
doesn't pass through MAKEFLAGS.

Test: Manual $(warnings $(origin MYTEST) $(MYTEST)) with old and new Kati
Change-Id: I5e4ffa66b3539a539cc7a26da24645b11445c469