Update MCUboot version to latest

Pick up latest version which contains all features
which were upstreamed from TF-M MCUboot fork.

Change-Id: I11bca8dc2067a6467656a8d987577299e8c0b7ff
Signed-off-by: Tamas Ban <tamas.ban@arm.com>
diff --git a/tf-m-build-docs-nightly.yaml b/tf-m-build-docs-nightly.yaml
index bb68c44..5adc156 100644
--- a/tf-m-build-docs-nightly.yaml
+++ b/tf-m-build-docs-nightly.yaml
@@ -55,8 +55,9 @@
           name: 'CODE_REPO'
           default: 'https://review.trustedfirmware.org/TF-M/trusted-firmware-m'
       - string:
-          name: 'MCUBOOT_VERSION'
-          default: 'v1.6.0'
+          name: 'MCUBOOT_REFSPEC'
+          default: 'refs/heads/master'
+#         default: 'refs/tags/v1.6.0'
       - string:
           name: 'MCUBOOT_URL'
           default: 'https://git.trustedfirmware.org/mirror/mcuboot.git'