Rework safe mode and have heap overwrite trigger it.

This creates a common safe mode mechanic that ports can share.
As a result, the nRF52 now has safe mode support as well.

The common safe mode adds a 700ms delay at startup where a reset
during that window will cause a reset into safe mode. This window
is designated by a yellow status pixel and flashing the single led
three times.

A couple NeoPixel fixes are included for the nRF52 as well.

Fixes #1034. Fixes #990. Fixes #615.

1 files changed, 2 insertions, 0 deletions

diff --git a/py/obj.c b/py/obj.c
index c20b549a3..fb59eec82 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -488,6 +488,8 @@ mp_obj_t mp_obj_subscr(mp_obj_t base, mp_obj_t index, mp_obj_t value) {
     mp_obj_type_t *type = mp_obj_get_type(base);
     if (type->subscr != NULL) {
         mp_obj_t ret = type->subscr(base, index, value);
+        // May have called port specific C code. Make sure it didn't mess up the heap.
+        assert_heap_ok();
         if (ret != MP_OBJ_NULL) {
             return ret;
         }