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/logging.yml

@@ -0,0 +1,13 @@
+# Python logging configuration to write all warnings to a separate file
+version: 1
+
+handlers:
+  warnings:
+    class: logging.FileHandler
+    level: WARNING
+    filename: warnings.log
+    formatter: BitBake.logfileFormatter
+
+loggers:
+  BitBake:
+    handlers: [warnings]