extmod/crypto-algorithms/sha256.c was recently fixed so that it didn't
include this header.

Prior to making this a config option it was previously available on these
(and all other) ports, and it makes sense to keep it enabled for mpy-cross
as well as ports that have a decent amount of space for the code.

Sometimes when setting a channel callback the callback fires immediately,
even if the compare register is set to a value far into the future. This
happens when the free running counter has previously been equal to what
happens to be in the compare register.

This patch make sure that there is no pending interrupt when setting a
callback.

This should be a little more efficient (since we anyway scan the input
packet for the interrupt char), and it should also fix any non-atomic read
issues with the buffer state being changed during an interrupt.

Throughput tests show that RX rate is unchanged by this patch.

The previous timeout value of 150ms could lead to data being lost (ie never
received by the host) in some rare cases, eg when the host is under load.
A value of 500ms is quite conservative and allows the host plenty of time
to read our data.

It can still be overwritten by a port in mpconfigport.h but for almost
all cases one can use the provided default.

This follows the pattern of how all other headers are now included, and
makes it explicit where the header file comes from.  This patch also
removes -I options from Makefile's that specify the mp-readline/timeutils/
netutils directories, which are no longer needed.

Such constants are MCU specific so shouldn't be specified in the board
config file (else it leads to too much duplication of code).

This patch also adds I2C timing values for the F767/F769 for 100k, 400k
and 1MHz I2C bus frequencies.

Not all F7 MCUs have SDMMC2.

By default the SDIO (F4) or SDMMC1 (L4, F7) is used as the SD card
peripheral, but if a board config defines MICROPY_HW_SDMMC2_CK and other
pins then the SD card driver will use SDMMC2.

With the existing timeout of 100ms the transfer would end prematurely if
the baudrate was low and the number of bytes to send was high.  This patch
fixes the problem by making the timeout proportional to the number of bytes
that are being transferred.

It's needed by the DMA driver to do an efficient reinitialisation.  This
patch follows what is done in the F4 HAL.

On MCUs with a cache (eg F7) this must be done or else the SPI data that is
transferred is incorrect.

The C nearbyint function has exactly the semantics that Python's round()
requires, whereas C's round() requires extra steps to handle rounding of
numbers half way between integers.  So using nearbyint reduces code size
and potentially eliminates any source of errors in the handling of half-way
numbers.

Also, bare-metal implementations of nearbyint can be more efficient than
round, so further code size is saved (and efficiency improved).

nearbyint is provided in the C99 standard so it should be available on all
supported platforms.

The -ansi flag is used for C dialect selection and it is equivalent to -std=c90.
Because it goes right before -std=gnu99 it is ignored as for conflicting flags
GCC always uses the last one.

When locking/unlocking a mutex we only need to protect against a thread
switch, not general interrupts.

This way, Timer and ExtInt callbacks can interrupt the low-priority
pendsv handler (for example thread switching).

We can actually handle interrupts during a thread switch (because we always
have a valid stack), but only if those interrupts don't access any of the
thread state (because the state may not correspond to the stack pointer).
So to be on the safe side we disable interrupts during the very short
period of the thread state+stack switch.

ExtInt, Timer and CAN IRQ callbacks are made to work with the scheduler.
They are still hard IRQs by default, but one can now call
micropython.schedule within the hard IRQ to schedule a soft callback.

The renames are:
HAL_Delay -> mp_hal_delay_ms
sys_tick_udelay -> mp_hal_delay_us
sys_tick_get_microseconds -> mp_hal_ticks_us

And mp_hal_ticks_ms is added to provide the full set of timing functions.

Also, a separate HAL_Delay function is added which differs slightly from
mp_hal_delay_ms and is intended for use only by the ST HAL functions.