CI: update for meta-zephyr changes

As of meta-zephyr dde88ba the layer is structured different, split into
meta-zephyr-core and meta-zephyr-bsp.

As we define our own machines for use with Zephyr, we can just use
meta-zephyr-core.

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

ci/meta-zephyr.yml

@@ -6,4 +6,5 @@ header:
 repos:
   meta-zephyr:
     url: https://git.yoctoproject.org/git/meta-zephyr
-    refspec: master
+    layers:
+      meta-zephyr-core:

ci/qemu-cortex-m3.yml

@@ -4,6 +4,11 @@ header:
     - ci/base.yml
     - ci/meta-zephyr.yml
 
+repos:
+  meta-zephyr:
+    layers:
+      meta-zephyr-bsp:
+
 local_conf_header:
   tclibc: |
     TCLIBC = "newlib"