py/stream: Remove mp_stream_errno and use system errno instead.

This change is made for two reasons:

1. A 3rd-party library (eg berkeley-db-1.xx, axtls) may use the system
   provided errno for certain errors, and yet MicroPython stream objects
   that it calls will be using the internal mp_stream_errno.  So if the
   library returns an error it is not known whether the corresponding errno
   code is stored in the system errno or mp_stream_errno.  Using the system
   errno in all cases (eg in the mp_stream_posix_XXX wrappers) fixes this
   ambiguity.

2. For systems that have threading the system-provided errno should always
   be used because the errno value is thread-local.

For systems that do not have an errno, the new lib/embed/__errno.c file is
provided.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/embed/__errno.c b/lib/embed/__errno.c
new file mode 100644
index 000000000..86417a02d
--- /dev/null
+++ b/lib/embed/__errno.c
@@ -0,0 +1,13 @@
+// This file provides a version of __errno() for embedded systems that do not have one.
+// This function is needed for expressions of the form: &errno
+
+static int embed_errno;
+
+#if defined(__linux__)
+int *__errno_location(void)
+#else
+int *__errno(void)
+#endif
+{
+    return &embed_errno;
+}