yamllint: fix all yamllint truthy errors

Fix all thruthy errors detected by yamllint:

yamllint -f parsable -c .yamllint $( find -regex '.*\.y[a]*ml' ) | \
  grep '(truthy)'

This only accepts true/false for boolean properties. Seems like python
takes all sort of formats:

https://github.com/yaml/pyyaml/blob/master/lib/yaml/constructor.py#L224-L235

But the current specs only mention "true" or "false"

https://yaml.org/spec/1.2.2/#10212-boolean

Which is the standard yamllint config.

Excluding codeconv and workflow files, as some are using yes/no instead
in the respective documentation.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

tests/boot/test_mcuboot/testcase.yaml

@@ -1,5 +1,5 @@
 common:
-    sysbuild: True
+    sysbuild: true
     harness: console
     harness_config:
       type: multi_line