As Zephyr currently doesn't handle MTU itself (ZEP-1998), limit amount
of data we send on our side.
Also, if we get unsuccessful result from net_nbuf_append(), calculate
how much data it has added still. This works around ZEP-1984.
Internal structure of k_fifo changed between 1.7 and 1.8, so we need
to abstract it away. This adds more functions than currently used, for
future work.
Without this, if there's a large chunk of data coming from hardware (e.g.
clipboard paste, or fed programmatically from the other side of the console),
there's a behavior of initial mass fill-in of the buffer without any
consumption, which starts much later and doesn't catch up with further
filling, leading to buffer overflow.
The foundation of recv() support is per-socket queue of incoming packets,
implemented using Zephyr FIFO object. This patch implements just recv()
for UDP, because TCP recv() requires much more fine-grained control of
network fragments and handling other issues, like EOF condition, etc.
Build happens in 3 stages:
1. Zephyr config header and make vars are generated from prj.conf.
2. libmicropython is built using them.
3. Zephyr is built and final link happens.
Minimal config can be now build with:
./make-minimal BOARD=...
This is required because of Makefile.exports magic, which in its turn depends
on PROJ_CONF to be set correctly at the beginning of Makefile parsing at all
times. Instead of adding more and more workarounds for that, it's better to
just move minimal support to a separate wrapper.
Also, remove Zephyr 1.5 era cruft from Makefile, and add support for Zephyr's
"run" target which supercedes older "qemu" target in upstream.
This is a typical problem with make: we want to trigger rebuilds only
if file actually changed, not if its timestamp changed. In this case,
it's aggravated by the fact that prj.conf depends on the value of
BOARD variable, so we need to do some tricks anyway. We still don't
try to detect if just BOARD changed, just try to generate new
prj.conf.tmp every time (quick), but do actual replacement of prj.conf
only if its content changed.
Mostly intended to ease experimentation, no particular plans for APIs
so far (far less their stability), is_preempt_thread() provided is
mostly an example.
