update 404'd source.android.com URLs

Change-Id: If305c7f6f3ac3624568de258bdc13b142ad32cf9
diff --git a/core/main.mk b/core/main.mk
index 8a86aeb..8c87c74 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -35,7 +35,7 @@
 $(warning ********************************************************************************)
 $(warning *  You are using version $(MAKE_VERSION) of make.)
 $(warning *  You must upgrade to version 3.81 or greater.)
-$(warning *  see http://source.android.com/download)
+$(warning *  see http://source.android.com/source/download.html)
 $(warning ********************************************************************************)
 $(error stopping)
 endif
@@ -109,7 +109,7 @@
 $(info The correct version is: 1.6.)
 $(info $(space))
 $(info Please follow the machine setup instructions at)
-$(info $(space)$(space)$(space)$(space)http://source.android.com/download)
+$(info $(space)$(space)$(space)$(space)http://source.android.com/source/download.html)
 $(info ************************************************************)
 $(error stop)
 endif
@@ -125,7 +125,7 @@
 $(info The correct version is: 1.6.)
 $(info $(space))
 $(info Please follow the machine setup instructions at)
-$(info $(space)$(space)$(space)$(space)http://source.android.com/download)
+$(info $(space)$(space)$(space)$(space)http://source.android.com/source/download.html)
 $(info ************************************************************)
 $(error stop)
 endif