Add initial FAT support

[utzig]

This adds initial FAT support currently relying on hal_flash (tested only on native).

Added a simple test app to apps/fat2native which reads an image file and does some basic operations.

Testing was done with this image file:

http://utzig.org/files/fatfs.img

Sample output:

FAT type = FAT12
Number of FATs = 2
Root DIR entries = 512
Sectors/FAT = 2
Volume start = 0
FAT start = 1
DIR start = 5
Data start = 37

    1004 KiB total drive space.
     994 KiB available.

Listing 0:/
FSEVEN~1		       0 bytes
README.TXT		      26 bytes

README.txt found, showing contents:

------------------------------------------------
This is a FAT image file.

------------------------------------------------

[utzig]

This is not meant to be merged yet!

[utzig]

In general this is good for merging now.

Left FIXME/TODOs in some of the files to work on later. Most important issue would probably be making FAT safe for multi-threading and multi-file operation (remove globals, add locking, etc).

[ccollins476ad]

Thanks Fabio, it looks great! I will merge it shortly.

P.S.: Should have called it fabfs :)