* 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.
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