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/Makefile b/core/Makefile
index d813ab7..a6974a3 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1299,7 +1299,7 @@
 
 # -----------------------------------------------------------------
 # The emulator package
-
+ifeq ($(BUILD_EMULATOR),true)
 INTERNAL_EMULATOR_PACKAGE_FILES += \
         $(HOST_OUT_EXECUTABLES)/emulator$(HOST_EXECUTABLE_SUFFIX) \
         prebuilts/qemu-kernel/$(TARGET_ARCH)/kernel-qemu \
@@ -1315,6 +1315,7 @@
 	@echo "Package: $@"
 	$(hide) zip -qj $@ $(INTERNAL_EMULATOR_PACKAGE_FILES)
 
+endif
 # -----------------------------------------------------------------
 # Old PDK stuffs, retired
 # The pdk package (Platform Development Kit)