Adds platform-independent interface for sleeping. (#577)

The intention is to replace all usage of deprecated POSIX function `usleep` with the function defined in this cross-platform header. Then different platforms can provide their implementation.

This will allow us to remove the feature macro hacks that we had to put in to make the code compile on recent releases of armgcc, such as `#define _BSD_SOURCE` or `#define _DEFAULT_SOURCE`. These are harmful, because they often conflict with each other and sometimes with expectations from middleware libraries (e.g. a macro declaring C99 source).

===

* Adds platform-independent interface for sleeping.

* Fix typo

src/os/sleep.h

@@ -0,0 +1,52 @@
+/** \copyright
+ * Copyright (c) 2021, Balazs Racz
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are  permitted provided that the following conditions are met:
+ * 
+ *  - Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ *  - 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.
+ *
+ * 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 HOLDER 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.
+ *
+ * \file sleep.h
+ * 
+ * Declares cross-platform sleep functions.
+ *
+ * @author Balazs Racz
+ * @date 10 Sep 2021
+ */
+
+#ifndef _OS_SLEEP_H_
+#define _OS_SLEEP_H_
+
+#include <inttypes.h>
+
+/// Sleep a given number of microseconds. The granularity of sleep depends on
+/// the operating system, for FreeRTOS sleeping less than 1 msec is not
+/// possible with this function.
+/// @param microseconds how long to sleep.
+static void microsleep(uint32_t microseconds) __attribute__((weakref("usleep")));
+
+
+/// Executes a busy loop for a given amount of time. It is recommended to use
+/// this only for small number of microseconds (e.g. <100 usec).
+/// @param microseconds how long to delay.
+extern void microdelay(uint32_t microseconds);
+
+#endif // _OS_SLEEP_H_