add exception to be able to build emulator_tests in conjunction with the
sdk.

Change-Id: Ic4c63c756c78da97657e08faa5b4b6891b951b03
diff --git a/core/main.mk b/core/main.mk
index ca10610..1eaf8a6 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -398,7 +398,7 @@
 sdk_repo_goal := $(strip $(filter sdk_repo,$(MAKECMDGOALS)))
 MAKECMDGOALS := $(strip $(filter-out sdk_repo,$(MAKECMDGOALS)))
 
-ifneq ($(words $(filter-out $(INTERNAL_MODIFIER_TARGETS) checkbuild,$(MAKECMDGOALS))),1)
+ifneq ($(words $(filter-out $(INTERNAL_MODIFIER_TARGETS) checkbuild emulator_tests,$(MAKECMDGOALS))),1)
 $(error The 'sdk' target may not be specified with any other targets)
 endif