Add function to check for ISR context

Add the function core_util_in_isr() so code can determine if it is
running in the context of an ISR.

platform/mbed_critical.c

@@ -37,6 +37,22 @@ bool core_util_are_interrupts_enabled(void)
 #endif
 }
 
+bool core_util_is_isr_active(void)
+{
+#if defined(__CORTEX_A9)
+    switch(__get_CPSR() & 0x1FU) {
+        case MODE_USR:
+        case MODE_SYS:
+            return false;
+        case MODE_SVC:
+        default:
+            return true;
+    }
+#else
+    return (__get_IPSR() != 0U);
+#endif
+}
+
 MBED_WEAK void core_util_critical_section_enter(void)
 {
     bool interrupts_disabled = !core_util_are_interrupts_enabled();

platform/mbed_critical.h

@@ -41,6 +41,17 @@ extern "C" {
   */
 bool core_util_are_interrupts_enabled(void);
 
+/** Determine if this code is executing from an interrupt
+  *
+  * This function can be called to determine if the code is running on interrupt context.
+  * @note
+  * NOTE:
+  * This function works for both cortex-A and cortex-M, although the underlyng implementation
+  * differs.
+  * @return true if in an isr, false otherwise
+  */
+bool core_util_is_isr_active(void);
+
 /** Mark the start of a critical section
   *
   * This function should be called to mark the start of a critical section of code.