No complex numbers though.

This file contains various MicroPython-specific helper functions, so isn't
good fit for lib/libc/.

Scenario: module1 depends on some common file from lib/, so specifies it
in its SRC_MOD, and the same situation with module2, then common file
from lib/ eventually ends up listed twice in $(OBJ), which leads to link
errors.

Make is equipped to deal with such situation easily, quoting the manual:
"The value of $^ omits duplicate prerequisites, while $+ retains them and
preserves their order." So, just use $^ consistently in all link targets.

Generally, ports should inherit INC from py.mk, append to it, not
overwrite it. TODO: Likely should do the same for other vars too.

MicroPython "network" module interface requires it to contains classes
to instantiate. But as we have a static network interace, make WLAN()
"constructor" just return module itself, and just make all methods
module-global functions.

Currently implements only .uname()

 * UDP currently not supported
 * As there is no way (that I know of) the espconn_regist_connectcb()
   callback can recognize on which socket has the connection arrived,
   only one listening function at a time is supported

Now all ports can use pybstdio.c to provide sys.stdin/stdout/stderr, so
long as they implement mp_hal_stdin_* and mp_hal_stdout_* functions.

Just always keep jumper in bootloader position. After flashing, uPy
automatically executed. And to get back to bootloader, do "make reset".

So far implements .scan(lambda x: print(x)) function to scan for WiFi access
points.

When esp-open-sdk is built with STANDALONE=y (the default) then ESP_SDK
is set to the correct value, so that "make" just works.

Some rodata items can go in iram/irom segment, but not others.  With
this patch ESP now has 24256 bytes of heap ram.  It passes 228 out of
248 tests from tests/basics directory.