Create systemtarball either with bz2 or gz

Change-Id: I64e93872942e8dcad4a894a906be7912bb6da308
Signed-off-by: Bruce Beare <brucex.j.beare@intel.com>
diff --git a/core/Makefile b/core/Makefile
index 628a8de..a2b9391 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -720,8 +720,12 @@
 		$(INSTALLED_SYSTEMTARBALL_TARGET)
 endef
 
+ifndef SYSTEM_TARBALL_FORMAT
+    SYSTEM_TARBALL_FORMAT := bz2
+endif
+
 system_tar := $(PRODUCT_OUT)/system.tar
-INSTALLED_SYSTEMTARBALL_TARGET := $(system_tar).bz2
+INSTALLED_SYSTEMTARBALL_TARGET := $(system_tar).$(SYSTEM_TARBALL_FORMAT)
 $(INSTALLED_SYSTEMTARBALL_TARGET): PRIVATE_SYSTEM_TAR := $(system_tar)
 $(INSTALLED_SYSTEMTARBALL_TARGET): $(FS_GET_STATS) $(INTERNAL_SYSTEMIMAGE_FILES)
 	$(build-systemtarball-target)