Based on the original patch by Galen Hazelwood:
https://github.com/micropython/micropython/pull/1517 .

This is required to write structures to files, pass to FFI functions,
etc.

Addresses issue #1529.

Also make sleep.c self-contained by moving initialization code,
instead of having part of the code in init.c, and add a header file
to accomodate this.
msec_sleep() now uses the usleep() implementation as well.

Fixes issues like #1532

They return None if no data available.

mpconfigport.h is a private, partial header not providing correct settings
unless included by py/mpconfig.h.

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.

This is required to properly select among overloaded methods. It however
relies on java.lang.Object-overloaded method to come last, which appears
to be the case for OpenJDK.

For example, don't pass Integer to double method. This is still not
selective enough to choose the right overloaded method maong those
taking objects.

Incluides several improvements and a few API changes to comply
with the new hardware API.

The UART REPL it's not enabled by default anymore.