Many includes of fat.h are pointless. Some includes are just for
SECTOR_SIZE. Add a file 'firmware/include/fs_defines.h' for that
and to define tuneable values that were scattered amongst various
headers.
Remove some local definitions of SECTOR_SIZE since they have to be
in agreement with the rest of the fs code anyway.
(We'll see what's in fact pointless in a moment ;)
Change-Id: I9ba183bf58bd87f5c45eba7bd675c7e2c1c18ed5
