Update MCUboot version to v1.9.0

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: I0c1797984ba8aeb721cf715bdfc8ade59927a3a6
diff --git a/tf-m-nightly.yaml b/tf-m-nightly.yaml
index f55cb46..af8a42a 100644
--- a/tf-m-nightly.yaml
+++ b/tf-m-nightly.yaml
@@ -84,7 +84,7 @@
           description: 'Reference configurations whose metrics will be sent to a SQUAD Dashboard'
       - string:
           name: 'MCUBOOT_REFSPEC'
-          default: 'v1.8.0'
+          default: 'v1.9.0'
           description: |
             Specify which MCUboot version to use
             refs/heads/master    : Pick up latest master