Update MCUboot repo and branch

MCUboot was moved to mcu-tools org, and now uses "main" instead of
"master" branch.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/repository.yml b/repository.yml
index d88c444..aa77325 100644
--- a/repository.yml
+++ b/repository.yml
@@ -110,8 +110,9 @@
 
     mcuboot:
         type: github
-        user: JuulLabs-OSS
+        user: mcu-tools
         repo: mcuboot
+        branch: main
         vers:
             master: 0-dev
             mynewt_1_7_0_tag: 1.3.1