Add atomic_flag utility

An atomic flag primitive is sometimes wanted, and it is cumbersome to
create it from the compare-and-swap operation - cumbersome enough that
people often don't bother.

Put in a core_util_atomic_flag that follows the C11/C++11 atomic_flag
API, such that it could be mapped to it with #define later.

platform/mbed_critical.c

@@ -100,6 +100,11 @@ void core_util_critical_section_exit(void)
     }
 }
 
+void core_util_atomic_flag_clear(volatile core_util_atomic_flag *flagPtr)
+{
+    flagPtr->_flag = false;
+}
+
 #if MBED_EXCLUSIVE_ACCESS
 
 /* Supress __ldrex and __strex deprecated warnings - "#3731-D: intrinsic is deprecated" */
@@ -107,6 +112,15 @@ void core_util_critical_section_exit(void)
 #pragma diag_suppress 3731
 #endif
 
+bool core_util_atomic_flag_test_and_set(volatile core_util_atomic_flag *flagPtr)
+{
+    uint8_t currentValue;
+    do {
+        currentValue = __LDREXB(&flagPtr->_flag);
+    } while (__STREXB(true, &flagPtr->_flag));
+    return currentValue;
+}
+
 bool core_util_atomic_cas_u8(volatile uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_t desiredValue)
 {
     do {
@@ -204,6 +218,15 @@ uint32_t core_util_atomic_decr_u32(volatile uint32_t *valuePtr, uint32_t delta)
 
 #else
 
+bool core_util_atomic_flag_test_and_set(volatile core_util_atomic_flag *flagPtr)
+{
+    core_util_critical_section_enter();
+    uint8_t currentValue = flagPtr->_flag;
+    flagPtr->_flag = true;
+    core_util_critical_section_exit();
+    return currentValue;
+}
+
 bool core_util_atomic_cas_u8(volatile uint8_t *ptr, uint8_t *expectedCurrentValue, uint8_t desiredValue)
 {
     bool success;

platform/mbed_critical.h

@@ -89,6 +89,43 @@ void core_util_critical_section_exit(void);
  */
 bool core_util_in_critical_section(void);
 
+/**
+ * A lock-free, primitive atomic flag.
+ *
+ * Emulate C11's atomic_flag. The flag is initially in an indeterminate state
+ * unless explicitly initialised with CORE_UTIL_ATOMIC_FLAG_INIT.
+ */
+typedef struct core_util_atomic_flag {
+    uint8_t _flag;
+} core_util_atomic_flag;
+
+/**
+ * Initialiser for a core_util_atomic_flag.
+ *
+ * Example:
+ * ~~~
+ *     core_util_atomic_flag in_progress = CORE_UTIL_ATOMIC_FLAG_INIT;
+ * ~~~
+ */
+#define CORE_UTIL_ATOMIC_FLAG_INIT { 0 }
+
+/**
+ * Atomic test and set.
+ *
+ * Atomically tests then sets the flag to true, returning the previous value.
+ *
+ * @param  flagPtr Target flag being tested and set.
+ * @return         The previous value.
+ */
+bool core_util_atomic_flag_test_and_set(volatile core_util_atomic_flag *flagPtr);
+
+/**
+ * Atomic clear.
+ *
+ * @param  flagPtr Target flag being cleared.
+ */
+void core_util_atomic_flag_clear(volatile core_util_atomic_flag *flagPtr);
+
 /**
  * Atomic compare and set. It compares the contents of a memory location to a
  * given value and, only if they are the same, modifies the contents of that