Really mark sdk_addon as PHONY to fix `m snod`

When we're using a dont_bother_goals (including snod), we don't load
build/make/core/tasks/*.mk, so sdk_addon isn't defined as a PHONY rule.

This causes a build break because build/make/core/Makefile uses
dist-for-goals with sdk_addon. dist-for-goals requires its arguments to
be PHONY targets.

Test: m snod
Change-Id: I69937fe9360be57766d17ed4c28b5d2a1e348308