px4_arch: add hw description constexpr util methods

These will be used in the board configurations.

platforms/common/include/px4_platform_common/constexpr_util.h

@@ -0,0 +1,59 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2019 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+#pragma once
+
+static inline int _constexpr_assert_failure(const char *msg)
+{
+	// we do 2 things that the compiler will refuse to execute at compile-time
+	// (and therefore trigger a compilation error):
+	// - define a local static variable
+	// - declare the method as non constexpr
+	static int i = 0;
+	return i;
+}
+
+/**
+ * Assertion that fails compilation if used in a constexpr context (that is executed at
+ * compile-time).
+ *
+ * Important: you need to ensure the code is executed at compile-time, e.g. by
+ * assigning the returned value of a constexpr method (where the assert is used)
+ * to a variable marked as constexpr. Otherwise the compiler might silently move
+ * execution to runtime.
+ *
+ * If executed at runtime, it has no effect other than slight runtime overhead.
+ */
+#define constexpr_assert(expr, msg) if (!(expr)) { _constexpr_assert_failure(msg); }
+
+