This patch forces a board to explicitly define TEXT1_ADDR in order to
split the firmware into two separate pieces.  Otherwise the default is now
to produce only a single continuous firmware image with all ISR, text and
data together.

To make it clearer that these addresses are both for firmware text and that
they have a prescribed ordering.

This patch allows a particular board to independently specify the linker
scripts for 1) the MCU memory layout; 2) how the different firmware
sections are arranged in memory.  Right now all boards follow the same
layout with two separate firmware section, one for the ISR and one for the
text and data.  This leaves room for storage (filesystem data) to live
between the firmware sections.

The idea with this patch is to accommodate boards that don't have internal
flash storage and only need to have one continuous firmware section.  Thus
the common.ld script is renamed to common_ifs.ld to make explicit that it
is used for cases where the board has internal flash storage.

Explicitly writing out the implementation of sys_tick_has_passed makes
these bdev files independent of systick.c and more reusable as a general
component.  It also reduces the code size slightly.

The irq.h header is added to spibdev.c because it uses declarations in that
file (irq.h is usually included implicitly via mphalport.h but not always).

Taking the address assumes that the pin is an object (eg a struct), but it
could be a literal (eg an int).  Not taking the address makes this driver
more general for other uses.

genhdr/pins.h is an internal header file that defines all of the pin
objects and it's cleaner to have pin.h include it (where the struct's for
these objects are defined) rather than an explicit include by every user.

The HAL requires strict aliasing optimisation to be turned on to function
correctly (at least for the SD card driver on F4 MCUs).  This optimisation
was recently disabled with the addition of H7 support due to the H7 HAL
having errors with the strict aliasing optimisation enabled.  But this is
now fixed in the latest stm32lib and so the optimisation can now be
re-enabled.

Thanks to @chuckbook for finding that there was a problem with the SD card
on F4 MCUs with the strict aliasing optimisation disabled.

There's no uSD Transceiver on this NUCLEO board.

Found the timing for full (400 KHz) and FM+ (1MHz) in the HAL examples, and
used CubeMX to calculate the standard value (100KHz).

This API matches (as close as possible) how other pyb classes allow inplace
operations, such as pyb.SPI.recv(buf).

This seems to reduce the Travis build time by roughly 1 minute / 10%.

The CMSIS files for the STM32 range provide macros to distinguish between
the different MCU series: STM32F4, STM32F7, STM32H7, STM32L4, etc.  Prefer
to use these instead of custom ones.

By using pre-compiled regexs, using startswith(), and explicitly checking
for empty lines (of which around 30% of the input lines are), automatic
qstr extraction is speed up by about 10%.

Casting the Data array to a uint32_t* leads to strict aliasing errors on
older gcc compilers.

This is useful for monitoring errors on the bus and knowing when a restart
is needed.

This patch provides a custom (and simple) function to receive data on the
CAN bus, instead of the HAL function.  This custom version calls
mp_handle_pending() while waiting for messages, which, among other things,
allows to interrupt the recv() method via KeyboardInterrupt.

This board allows to test CAN support on the L4 series.

This is instead of returning an empty bytes object, and matches how other
ports handle non-blocking UART read behaviour.

mp_spiflash_read had a bug in it where "dest" and "addr" were incremented
twice for a certain special case.  This was fixed, which then allowed the
function to be simplified to reduce code size.

mp_spiflash_write had a bug in it where "src" was not incremented correctly
for the case where the data to be written included the caching buffer as
well as some bytes after this buffer.  This was fixed and the resulting
code simplified.

If this function is used then objlist.h is already included to get the
definition of mp_obj_list_t.

This function was never used for unicode/utf8 handling code, or anything
else, so remove it to keep things clean.

Certain pins (eg 4 and 5) seem to behave differently at the hardware level
when in open-drain mode: they glitch when set "high" and drive the pin
active high for a brief period before disabling the output driver.  To work
around this make the pin an input to let it float high.

Using an explicit read eliminates the need to invalidate the D-cache after
enabling the memory mapping mode, which takes additional time.