Upgrade to latest FatFS (R0.14b)

[urish]

First of all, thanks for maintaining this library! It's very useful for me.

I've run into an issue where the filesystem created by f_mkfs() can't be read by Arduino's SdFat library. The reason for that is that f_mkfs() always creates a single copy of the FAT ,whereas SdFat expects to have 2 copies.

FatFS R0.14 added support for specifying the number of FAT copies in the options for f_mkfs() (the n_fat property of the opt parameter). This new parameter makes it possible to create FAT file systems that are compatible with the SdFat library.

If you need any additional input or some help with realizing the update (i.e. pull request), please let me know.