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) $(MYTEST)) with old and new Kati
Change-Id: I5e4ffa66b3539a539cc7a26da24645b11445c469
diff --git a/core/ninja.mk b/core/ninja.mk
index 04efcc1..f3aa70e 100644
--- a/core/ninja.mk
+++ b/core/ninja.mk
@@ -123,6 +123,14 @@
 NINJA_EXTRA_ARGS += -j$(NINJA_REMOTE_NUM_JOBS)
 else
 NINJA_MAKEPARALLEL := $(MAKEPARALLEL) --ninja
+
+# 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 ?=
+#
+# makeparallel already clears MAKEFLAGS, so it's not necessary in the GOMA case
+KATI_MAKEPARALLEL := MAKEFLAGS=
 endif
 
 NINJA_ARGS += $(NINJA_EXTRA_ARGS)