Update MCUBOOT to TF-Mv1.4-integ-rc2

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I8416677ea275231915ed0f522bbe0cf1939f9c69
diff --git a/tf-m-build-config.yaml b/tf-m-build-config.yaml
index 17652d5..31c77f4 100644
--- a/tf-m-build-config.yaml
+++ b/tf-m-build-config.yaml
@@ -93,7 +93,7 @@
           default: 'https://git.trustedfirmware.org/mirror/mbed-tls.git'
       - string:
           name: 'MCUBOOT_REFSPEC'
-          default: '4f8091318b4026d14af9e5a7036825bff62fb612'
+          default: 'TF-Mv1.4-integ-rc2'
           description: |
             Specify which MCUboot version to use
             refs/heads/master    : Pick up latest master