stmhal: Set entry point for ELF binary debugging

mode argument is used to assert it works
encoding argument is used to make sure CPython uses the correct encoding
as it does not automatically use utf-8

This makes open() and _io.FileIO() more CPython compliant.
The mode kwarg is fully iplemented.
The encoding kwarg is allowed but not implemented; mainly to allow
the tests to specify encoding for CPython, see #874

When loading the ELF binary to the board with a debugger, the debugger
needs to know at which point to start executing the code. Currently the
entry point defaults to the start of the .text section.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>

This fixed an issue with a certain SD card sometimes not initialising
first time round.  See issue #822 for related, and thanks to
@iabdalkader for the idea.

Also, usocket.readinto(). Known issue is that .readinto() should be available
only for binary files, but micropython uses single method table for both
binary and text files.

Addresses issue #915.

Addresses issue #912.

Just like they handled in other read*(). Note that behavior of readline()
in case there's no data when it's called is underspecified in Python lib
spec, implemented to behave as read() - return None.

This is CPython-compatible convention established yet in acb13886.

The code previously assumed that only functions can be closed over.

Previously, a realloc to a smaller memory chunk size would not free the
unused blocks in the tail of the chunk.

Remove zlibd, superceded by uzlib

Module "uzlib" - based on similarly named library

https://github.com/pfalcon/uzlib

os, time, select modules are now prefixed with u, but are still
available (via weak links) as their original names.

ure and ujson now available as re and json via weak links.

With this patch a port can enable module weak link support and provide
a dict of qstr->module mapping.  This mapping is looked up only if an
import fails to find the requested module in the filesystem.

This allows to have the builtin module named, eg, usocket, and provide
a weak link of "socket" to the same module, but this weak link can be
overridden if a file by the name "socket.py" is found in the import
path.

Viper can now do: ptr8(buf)[0], which loads a byte from a buffer using
machine instructions.