minor fix in A/B boot

Author: fermuch

uboot-config/uboot.env

@@ -27,7 +27,9 @@ bootpart=1:1
 # ---------- Nerves firmware state ---------------------------
 #
 nerves_fw_active=a
-nerves_fw_autovalidate=1
+# Autovalidate must be 0 so that firmware only gets validated explicitly
+# by the running system. This ensures A/B failover works correctly.
+nerves_fw_autovalidate=0
 nerves_fw_validated=1
 nerves_fw_booted=0