Add support for storing args during an exception raised by an irq.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

1 files changed, 18 insertions, 0 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index 99d697f9ad..911125c8a4 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -163,6 +163,16 @@
 #define MICROPY_STACK_CHECK (1)
 #endif
 
+// Whether to have an emergency exception buffer
+#ifndef MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
+#define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF (0)
+#endif
+#if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
+#   ifndef MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE
+#   define MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE (0)   // 0 - implies dynamic allocation
+#   endif
+#endif
+
 // Whether to include REPL helper function
 #ifndef MICROPY_HELPER_REPL
 #define MICROPY_HELPER_REPL (0)
@@ -378,6 +388,14 @@ typedef double mp_float_t;
 /*****************************************************************************/
 /* Miscellaneous settings                                                    */
 
+// On embedded platforms, these will typically enable/disable irqs.
+#ifndef MICROPY_BEGIN_ATOMIC_SECTION
+#define MICROPY_BEGIN_ATOMIC_SECTION()
+#endif
+#ifndef MICROPY_END_ATOMIC_SECTION
+#define MICROPY_END_ATOMIC_SECTION()
+#endif
+
 // Allow to override static modifier for global objects, e.g. to use with
 // object code analysis tools which don't support static symbols.
 #ifndef STATIC