Fix MBED_ASSERT for UTs

For UTs mbed_assert_internal should not be declared as MBED_NORETURN
as UT stub for mbed_assert_internal only prints out the assert trace
and returns back to original code.

UNITTESTS/target_h/platform/mbed_assert.h

@@ -39,7 +39,12 @@ extern "C" {
  *  @param file File where assertation failed.
  *  @param line Failing assertation line number.
  */
-MBED_NORETURN void mbed_assert_internal(const char *expr, const char *file, int line);
+// mbed_assert_internal UT stub only prints an assert trace and returns, so therefore
+// MBED_NORETURN must not be defined for UTs.
+#ifndef UNITTEST
+MBED_NORETURN
+#endif
+void mbed_assert_internal(const char *expr, const char *file, int line);
 
 #ifdef __cplusplus
 }
@@ -132,4 +137,3 @@ do {                                                     \
 /**@}*/
 
 /**@}*/
-