Adds support for 3 Cortex-M boards, selectable via "BOARD" in the Makefile:
- microbit, Cortex-M0 via nRF51822
- netduino2, Cortex-M3 via STM32F205
- mps2-an385, Cortex-M3 via FPGA

netduino2 is the default board because it's supported by older qemu
versions (down to at least 2.5.0).

Instead of checking each callback (currently storage and dma) explicitly
for each SysTick IRQ, use a simple circular function table indexed by the
lower bits of the millisecond tick counter.  This allows callbacks to be
easily enabled/disabled at runtime, and scales well to a large number of
callbacks.

This is a good board to demonstrate the use of Mboot because it only has a
USB HS port exposed so the native ST DFU mode cannot be used.  With Mboot
this port can be used.

If a custom bootloader is enabled (eg mboot) then machine.bootloader() will
now enter that loader.  To get the original ST DFU loader pass any argument
to the function, like machine.bootloader(1).

Also enable CONFIG_SPIRAM_IGNORE_NOTFOUND to allow boards with faulty or
missing SPIRAM to still boot.

Don't exclude the Timer instance 1 entry from machine_timer_obj[] when
using soft PWM. The usage is already checked when creating the Timer,
so just create an empty entry.

It's off by default and can be enabled at run-time with:

    pyb.USB_VCP().init(flow=pyb.USB_VCP.RTS)

Code size change:
nrf51: -44
nrf52: -52

Code size reductions:
nrf51: -132
nrf52: -188

If needed these parameters can be added back and made functional one at a
time.  It's better to explicitly not support them than to silently allow
but ignore them.

This saves a bit of code:
nrf51: -176
nrf52: -152

Python defines warnings as belonging to categories, where category is a
warning type (descending from exception type). This is useful, as e.g.
allows to disable warnings selectively and provide user-defined warning
types.  So, implement this in MicroPython, except that categories are
represented just with strings.  However, enough hooks are left to implement
categories differently per-port (e.g. as types), without need to patch each
and every usage.

With clock bypass enabled the attached SD card is clocked at the maximum
48MHz.  But some SD cards are unreliable at these rates.  Although it's
nice to have high speed transfers it's more important that the transfers
are reliable for all cards.  So disable this clock bypass option.

Enable in mpconfigboard.h via #define MBOOT_USB_AUTODETECT_USB (1).
Requires MICROPY_HW_USB_FS and MICROPY_HW_USB_HS to be enabled as well.

Fixes issue #4445.