sys: introduce coreclk utility function

sys/include/clk.h

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2021 Inria
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    sys_clk System core clock
+ * @ingroup     sys
+ * @{
+ *
+ * @file
+ * @brief       System core clock utility functions
+ */
+
+#ifndef CLK_H
+#define CLK_H
+
+#include <assert.h>
+#include <stdint.h>
+#include "periph_conf.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Get the current system core clock frequency in Hz.
+ *
+ * @returns current system core clock frequency in Hz
+ */
+static inline uint32_t coreclk(void) {
+#if defined(CLOCK_CORECLOCK)
+    return CLOCK_CORECLOCK;
+#else
+    extern uint32_t cpu_coreclk;
+    assert(cpu_coreclk != 0);
+    return cpu_coreclk;
+#endif
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* CLK_H */
+/** @} */