ci: Also syntax check YAML files

Now that we mostly work in YAML files rather than JSON, let's adapt our
checks accordingly.

.prow.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -xeuo pipefail
 
-yum -y install jq
+yum -y install jq PyYAML
 make syntax-check
 # And disable the container again until we figure out about Prow -> internal
 # make container

Makefile

@@ -17,6 +17,11 @@ syntax-check:
 		jq < $${jsonfile} . >/dev/null; \
 		echo "OK"; \
 	done
+	@set -e; for yamlfile in $$(find ${ROOT_DIR} -name '*.yaml' -o -name '*.yml'); do \
+		echo -n "Checking YAML syntax for $${yamlfile}... "; \
+		python -c "import yaml; yaml.safe_load(open('$${yamlfile}'))"; \
+		echo "OK"; \
+	done
 
 .PHONY: repo-refresh
 repo-refresh: