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/CleanSpec.mk b/CleanSpec.mk
index 74ad978..6d81733 100644
--- a/CleanSpec.mk
+++ b/CleanSpec.mk
@@ -125,6 +125,8 @@
 # Now we switched to build against Mac OS X SDK 10.6
 $(call add-clean-step, rm -rf $(OUT_DIR)/host/darwin-x86/obj)
 
+$(call add-clean-step, rm -f $(OUT_DIR)/versions_checked.mk)
+
 # ************************************************
 # NEWER CLEAN STEPS MUST BE AT THE END OF THE LIST
 # ************************************************