When write a file into FAT file system, it will search a match file in
root dir. So the find_directory_entry() will get the first cluster of
root dir content and search the directory item one by one. If the file
is not found, we will call get_fatent_value() to get next cluster of root
dir via lookup the FAT table and continue the search.

The issue is in FAT16/12 system, we cannot get root dir's next clust
from FAT table. The FAT table only be use to find the clust of data
aera in FAT16/12.

In FAT16/12 if the clust is in root dir, the clust number is a negative
number or 0, 1. Since root dir is located in front of the data area.
Data area start clust #2

. So the root dir clust number should < 2.

This patch will check above situation before call get_fatenv_value().
If curclust is < 2, include minus number, we just increase one on the
curclust since root dir is in continous cluster.

The patch also add a sanity check for entry in get_fatenv_value().

Signed-off-by: Josh Wu <josh.wu@atmel.com>