CI: use kirkstone branch, not master

Until we have released and branched meta-arm we're tracking kirkstone, not
master.  oe-core has branched and master is changing fast, so switch
the default branch now.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/base.yml

@@ -5,7 +5,7 @@ distro: poky
 
 defaults:
   repos:
-    refspec: master
+    refspec: kirkstone
 
 repos:
   meta-arm:

ci/clang.yml

@@ -4,6 +4,7 @@ header:
 repos:
   meta-clang:
     url: https://github.com/kraj/meta-clang
+    refspec: master
 
 local_conf_header:
   clang: |

ci/meta-virtualization.yml

@@ -6,3 +6,4 @@ header:
 repos:
   meta-virtualization:
     url: git://git.yoctoproject.org/meta-virtualization
+    refspec: master