Commit a380ce6e removed CONFIG_FAT_WRITE
from novena.h , so Novena lost ability to write FAT partitions. Add this
functionality back.

Signed-off-by: Marek Vasut <marex at denx.de>
Cc: Peter Robinson <pbrobinson at gmail.com>
Cc: Stefano Babic <sbabic at denx.de>