Similar to existing mp_hal_delay_ms/mp_hal_ticks_ms.

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.

These MPHAL functions are intended to replace previously used HAL_Delay(),
HAL_GetTick() to provide better naming and MPHAL separation (they are
fully equivalent otherwise).

Also, refactor extmod/modlwip to use them.

This gets uPy readline working with unix port, with tab completion and
history.  GNU readline is still supported, configure using
MICROPY_USE_READLINE variable.

Reference MCU is dsPIC33J256GP506 with 256k ROM and 8k RAM, on the dsPIC
DSC Starter Kit board.  The REPL works, GC works, pyb module has LED and
Switch objects.  It passes some tests from the test suite (most it can't
run because it doesn't have the Python features enabled).

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Instead of pyb.switch() as a function, it's more consistent (with
respect to all the other modules and peripherals) to have
pyb.Switch() create a switch object.  This then generalises to having
multiple switches.  Call the object to get its state.  Use sw.callback
to set the callback function for when the switch is pressed.

Can now choose at boot up whether the USB device is CDC+MSC or CDC+HID.
Choice is made by an option in boot.py, with default being CDC+MSC.
HID+MSC is not currently supported, but should be easy to implement.

Boot up now has ability to change the reset mode: hold down USR switch
while booting and LEDs will count from 1 to 7 to indicate the boot mode.
Release USR when correct mode is selected.  Current modes are 1 (normal
boot), 2 (safe mode), 3 (reset FS mode).

* Move user switch code from main.c into a separate module (usrsw)
* Add usrsw.c to Makefile