Create boottarball just like systemtarball

Change-Id: I97640bc973ab1d115499878a63bff48f80b326ec
Signed-off-by: Bruce Beare <brucex.j.beare@intel.com>
diff --git a/core/main.mk b/core/main.mk
index 14817c2..8a86aeb 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -659,6 +659,9 @@
 .PHONY: systemtarball
 systemtarball: $(INSTALLED_SYSTEMTARBALL_TARGET)
 
+.PHONY: boottarball
+boottarball: $(INSTALLED_BOOTTARBALL_TARGET)
+
 .PHONY: userdataimage
 userdataimage: $(INSTALLED_USERDATAIMAGE_TARGET)