ci: fail any build that emits warnings

Using a custom logging.yml we can instruct BitBake's logger to write all
warnings and errors into a separate log file.

Then after the build has finished we can see if the log file is empty and if
not show it and abort the build.

Change-Id: Ida835b5c822941fb513dfb1758b4ec195e0050fc
Signed-off-by: Ross Burton <ross.burton@arm.com>

kas/base.yml

@@ -25,6 +25,9 @@ repos:
     layers:
       meta-oe:
 
+env:
+  BB_LOGCONFIG: ""
+
 local_conf_header:
   base: |
     CONF_VERSION = "1"