Signed-off-by: Sven Wegener <sven.wegener@stealer.net>

This file contains various MicroPython-specific helper functions, so isn't
good fit for lib/libc/.

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.

Scenario: module1 depends on some common file from lib/, so specifies it
in its SRC_MOD, and the same situation with module2, then common file
from lib/ eventually ends up listed twice in $(OBJ), which leads to link
errors.

Make is equipped to deal with such situation easily, quoting the manual:
"The value of $^ omits duplicate prerequisites, while $+ retains them and
preserves their order." So, just use $^ consistently in all link targets.

Generally, ports should inherit INC from py.mk, append to it, not
overwrite it. TODO: Likely should do the same for other vars too.

This fix adds PIDs 9801 and 9802 to the pybcdc.inf file.

When in CDC only mode, it presents itself as a Communcations
device rather than as a composite device. Presenting as a
composite device with only the CDC interface seems to confuse
windows.

To test and make sure that the correct pybcdc.inf was being used,
I used USBDeview from http://www.nirsoft.net/utils/usb_devices_view.html
to uninstall any old pyboard drivers (Use Control-F and search
for pyboard). I found running USBDeview as administrator worked best.

Installing the driver in CDC+MSC mode first is recommended (since the
pybcdc.inf file in on the internal flash drive). Then when you switch
modes everything seems to work properly.

I used https://github.com/dhylands/upy-examples/blob/master/boot_switch.py
to easily switch the pyboard between the various USB modes for testing.

This needs to land afte #1407 lands, since #1407 is where
the CMSIS_MCU was defined.

This causes unnecessary constants to no longer be generated.
Some constants (like UART_BRR) are different between the F4 and F7

Thanks to Dave Hylands for the patch.

This is so that the DMA can be shared by multiple peripherals.

Each board now needs an mpconfigboard.mk file which defines AF_FILE and
LD_FILE.

Also moved stm32f405.ld to boards/ directory to keep things organised.

This can get PYBv1.0 stop current down to around 290uA.

This helps make files reusable across other ports.

The unused files are from the ST demos for different USB classes and are
not needed for the stmhal port.

Addresses issue #1081.

acoshf, asinhf, atanhf were added from musl.  mathsincos.c was
split up into its original, separate files (from newlibe-nano-2).
tan was added.

All of the important missing float functions are now implemented,
and pyboard now passes tests/float/math_fun.py (finally!).

This patch overhauls the network driver interface.  A generic NIC must
provide a set of C-level functions to implement low-level socket control
(eg socket, bind, connect, send, recv).  Doing this, the network and
usocket modules can then use such a NIC to implement proper socket
control at the Python level.

This patch also updates the CC3K and WIZNET5K drivers to conform to the
new interface, and fixes some bugs in the drivers.  They now work
reasonably well.

So can remove unnecessary stm32f4xx_hal_msp.c file.

Addresses issue #934.

Addresses issue #921.