This patch allows you to stop auto-indent by pressing enter on a second
blank line.  Easier than having to use backspace, and prevents new users
from getting stuck in auto-indent mode.

This patch adds/subtracts a constant from the 30-bit float representation
so that str/qstr representations are favoured: they now have all the high
bits set to zero.  This makes encoding/decoding qstr strings more
efficient (and they are used more often than floats, which are now
slightly less efficient to encode/decode).

Saves about 300 bytes of code space on Thumb 2 arch.

Initial power up also includes VBAT.

If LSE is configured but fails to start, LSI is used until next full power
cycle.  Also handles STM32F7xx variant.

The default setting of using the "highest" method available doesn't
work with some servers like Microsoft Azure. TLSV1 seems to work with
pretty much any server.

This fixes paste mode (Ctrl-E) which was not working for the
telnet REPL.

I left memzip in for the time being, so you can choose in
the Makefile whether to USE_FROZEN or USE_MEMZIP.

It looks like using frozen saves about 2472 bytes (using my
set of 15 python files), mostly due to overheads in the
zip file format.

- use correct 'mingw-w64' package name
- small grammar fixes
- modify Cygwin build instructions to use that same compiler as well: the
  original mingw is stuck at gcc v4.7 and does not seem to be updated anymore
- make it clear thet uPy also builds using Visual Studio versions > 2013

Reported here: http://forum.micropython.org/viewtopic.php?f=2&t=845

Ubuntu's mingw32 has gcc 4.2.1, which is rather old and has incorrect
non-initialized variable analysis which produces warnings, which
per MicroPython default settings get turned into errors.

Contains implementation of ?: (non-capturing groups), ?? (non-greedy ?),
as well as much improved robustness, and edge cases and error handling by
Amir Plivatsky (@ampli).

Otherwise for code like:

while (socket->incoming == NULL) {
    LWIP_DELAY(100);
}

a compiler may cache it in a register and it will be an infinite loop.

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

This makes format specifiers ~ fully compatible with CPython.

Adds 24 bytes for stmhal port (because previosuly we had to catch and report
it's unsupported to user).

This is the case already when using just subprocess.check_output, but in
the special cases (cmdline, meminfo, ...) the carriage return gets lost
during output processing so restore it in the end.
This fixes the micropython/meminfo.py test on Windows.

This fixes the basics/exception_chain.py test, also see #1500 for more
info and [4300c7db] where the same change was done for the other ports.

As per ARM convention.