libutil: Introduce multi-level message logging

The goal of util_log is to provide a facility for a multi-level message
logging on stderr. This allows to selectively enable/disable log messages
via a log level which can be adjusted at runtime.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

include/lib/util_log.h

@@ -0,0 +1,28 @@
+/**
+ * @defgroup util_log_h util_file: Multi-level message logging interface
+ * @{
+ * @brief Multi-level message logging
+ *
+ * Copyright IBM Corp. 2021
+ *
+ * s390-tools is free software; you can redistribute it and/or modify
+ * it under the terms of the MIT license. See LICENSE for details.
+ */
+
+#ifndef LIB_UTIL_LOG_H
+#define LIB_UTIL_LOG_H
+
+enum util_log_level {
+	UTIL_LOG_ERROR,
+	UTIL_LOG_WARN,
+	UTIL_LOG_INFO,
+	UTIL_LOG_DEBUG,
+	UTIL_LOG_TRACE,
+	UTIL_LOG_NUM_LEVELS	/* Must be the last one. */
+};
+
+void util_log_set_level(int log_level);
+
+void util_log_print(int log_level, const char *fmt, ...);
+
+#endif /** LIB_UTIL_LOG_H @} */