Fix build.

Set BUILD_EMULATOR to true when HOST_OS is linux.
Disable the emulator package target if BUILD_EMULATOR is not true.

Change-Id: I8987c0a091622baa0e861b451e635c4ddb148b29
diff --git a/core/main.mk b/core/main.mk
index 21e2e7f..64be9ff 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -187,7 +187,9 @@
 ifeq ($(strip $(wildcard $(MACOSX_SDK))),)
   BUILD_EMULATOR := false
 endif
-endif
+else   # HOST_OS is not darwin
+  BUILD_EMULATOR := true
+endif  # HOST_OS is darwin
 
 $(shell echo 'VERSIONS_CHECKED := $(VERSION_CHECK_SEQUENCE_NUMBER)' \
         > $(OUT_DIR)/versions_checked.mk)