Update makefiles from anywhere.

Test: with update-all-google-makefiles.sh
Change-Id: I794e74e706ae7666f5b332393edc7990193b40a1
diff --git a/update-makefiles.sh b/update-makefiles.sh
index fd82d6a..ee116d1 100755
--- a/update-makefiles.sh
+++ b/update-makefiles.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 
-source system/tools/hidl/update-makefiles-helper.sh
+source $ANDROID_BUILD_TOP/system/tools/hidl/update-makefiles-helper.sh
 
 do_makefiles_update \
     "android.frameworks:frameworks/hardware/interfaces" \