Only IPSR and BASEPRI special registers supported at the moment, but easy
to extend in the future.

uclibc objects call __GI_vsnprintf().

Oftentimes, libc, libm, etc. don't come compiled with CPU compressed code
option (Thumb, MIPS16, etc.), but we may still want to use such compressed
code for MicroPython itself.

At least it's defined as "unsiged". We don't try to support unicode still,
but at least apply workaround for DJGPP build.

Previously, sizeof() blindly assumed LAYOUT_NATIVE and tried to align
size even for packed LAYOUT_LITTLE_ENDIAN & LAYOUT_BIG_ENDIAN. As sizeof()
is implemented on a strucuture descriptor dictionary (not an structure
object), resolving this required passing layout type around.

This way mp_float_t can be changed to, eg, double.

Addresses issue #1697.

This is refactoring to enable support for the two USB PHYs available on
some STM32F4 processors to be used at the same time. The F405/7 & F429
have two USB PHYs, others such as the F411 only have one PHY.

This has been tested separately on a pyb10 (USB_FS PHY) and F429DISC
(USB_HS PHY) to be able to invoke a REPL/USB.  I have modified a PYBV10
to support two PHYs.

The long term objective is to support a 2nd USB PHY to be brought up as a
USB HOST, and possibly a single USB PHY to be OTG.

Addresses issue #1693.

Also prints a nicer error message if the serial connection could not be
established.

Not available for minimal build for example.

Currently nlr_jump_fail prints that there was an uncaught exception
but nothing about the exception.
This patch causes nlr_jump_failed to try to print the exception.
Given that printf was called on the line above, I think that
the call to mp_obj_print_exception has about as much likelyhood
of succeeding as the printf does.

When you use the USER button to perform a filesystem reset
at boot time then it wipes out the filesystem and creates
a new boot.py and main.py.  With this patch these files are
executed after formatting, ensuring that pyb and machine modules
get imported.

Automagically skip related modules.

Per CPython docs, "Registering a file descriptor that’s already registered
is not an error, and has the same effect as registering the descriptor
exactly once."
https://docs.python.org/3/library/select.html#select.poll.register

That's somewhat ambiguous, what's implemented here is that if fd si not
yet registered, it is registered. Otherwise, the effect is equivalent to
modify() method.

Based on similar usage for sys.argv/sys.path.

This for example will allow people to reload modules which didn't load
successfully (e.g. due to syntax error).

Usually this checking is done by VM on jump instructions, but for linear
sequences of instructions and builtin functions this won't happen. Particular
target of this change is long-running builtin functions like time.sleep().

This is a hack to free up TIM3 so that it can be used by the user.
Instead we use the PVD irq to call the USB VCP polling function, and
trigger it from SysTick (so SysTick itself does not do any processing).

The feature is enabled for pyboard lite only, since it lacks timers.

Consider the following scenario: SD card is being read by pyboard; USB
irq comes in for MSC read request; SD card needs to be read from within
USB irq while SD read is already ongoing.  Such contention needs to be
avoided.

This patch provides a simple solution, to raise the irq priority above
that of the USB irq during SD DMA transfers.  Pyboard and PC can now
read from the SD card at the same time (well, reads are interleaved).