For each iteration of polling BLE events from the Bluetooth LE stack.

Enabled by default on microbit targets, with or without BLE stack.  Also
enable softpwm to make display and music module compile.

Not all targets are using bluetooth le.

Applicable for targets with board specific modules.

Increases size by 68 bytes.  Should be considered to be removed as temp
module is already providing this functionality.

To use if BLE stack is enabled.  The custom linker script also set off
space enough to compile in microbitfs+hal_nvmc.

If the board has these drivers then they will be initialized:
- softpwm (implicit ticker)
- music module
- board specific module

Also update microbit_music_init0 to register low priority ticker callback
for the music module.

- Rename microbit_module_init to board_module_init0 which is the generic
  board module init function.
- Add low priority callback registration of display tick handler in the
  module init function.

- Rename init function to ticker_init0.
- Implement ticker_register_low_pri_callback (recycle of unused
  set_low_priority_callback function which was unimplemented).
- Add support for registering 2 low pri callbacks.  For now, one intended
  for microbit display, and one for modmusic.

So that users of the board module can find the init function of the module
implicitly.

And update the API to align with new unary/binary function callback
structures.

Makefile and mpconfigport.h update is generic, and could be used by other
boards to give extra modules which are only for a selected board.

This aligns implementation with new style structures.

Also rename modmicrobit.h to microbitconstimage.h.

Source: https://github.com/lancaster-university/microbit-dal.git

From micro:bit port repository, https://github.com/bbcmicrobit/micropython

Costs 136 bytes on a nRF51822.

Saves 448 bytes of flash. Can still be enabled using:

    make FROZEN_MPY_DIR=freeze BOARD=foo

Disconnecting after a connect would not restart advertising, so
reconnecting may get harder.

The ampy tool expects a "soft reboot" line when it does a soft reset.

Using libm from micropython free up about 5.5kb flash on nrf52
targets which have floating point enabled.

This saves about 80 bytes of code size.

When there is a non-BLE event (sd_evt_get), the ble_evt_handler is
invoked anyway even if it returns NRF_ERROR_NOT_FOUND.

* ports/nrf: Add micro:bit filesystem.

This filesystem has been copied from BBC micro:bit sources [1] and
modified to work with the nRF5x port.

[1]: https://github.com/bbcmicrobit/micropython/blob/master/source/microbit/filesystem.c

* ports/nrf/modules/uos: Make listdir() and ilistdir() consistent.

This removes the optional direcotry paramter from ilistdir(). This is
not consistent with VFS, but makes more sense when using only the
microbit filesystem.

Saves about 100 bytes.

* ports/nrf/modules/uos: Add code size comment.

  * Remove FLASH_ISR and merge .isr_vector into FLASH_TEXT. This saves
    some code space, especially on nRF52 devices.
  * Reserve space for nonvolatile storage of data. This is the place for
    a filesystem (to be added).

The code wasn't tested yet without a SoftDevice.

This has been tested and works.

Most boards don't have an SD card so it makes no sense to have it
enabled. It can be enabled per board (mpconfigboard.h).

This is only a library for flash access. Actual file system support will
be added later.

This saves about 17kB.

This saves about 6-7kB.