Update to v1.3.1

Makefile

@@ -15,7 +15,7 @@
 # Bump these on release - and please check ISO_VERSION for correctness.
 VERSION_MAJOR ?= 1
 VERSION_MINOR ?= 3
-VERSION_BUILD ?= 0
+VERSION_BUILD ?= 1
 # Default to .0 for higher cache hit rates, as build increments typically don't require new ISO versions
 ISO_VERSION ?= v$(VERSION_MAJOR).$(VERSION_MINOR).0