Initial FAT support

[Fabio Utzig <ut...@utzig.org>]

Hello,

I just opened a new PR adding FAT support:

https://github.com/apache/incubator-mynewt-core/pull/121

This implements: https://issues.apache.org/jira/browse/MYNEWT-318

I've used Elm-Chan's FATFS (kind of de-facto in the embedded world):
http://www.elm-chan.org/fsw/ff/00index_e.html 

This is basically 1-clause BSD compatible licensed so should be no
problem to add to repo but correct me if I'm wrong!

For now the implementation is limited to using hal_flash infrastructure.
Since no sane person would ever format a microcontroller flash using FAT
(I hope!), I see two options moving forward:

1) Write a MMC driver to access SD-Cards. I think this would rely only
on having a hal_spi driver.
2) Add support for USB and USB disks. This is much harder but will have
to be done eventually anyway.

So, what's the suggestion?

Fabio Utzig