fs/fatfs: Fix path handling is some functions
fs/fatfs: Fix path handling is some functions

File system paths starting with drive prefix were not
correctly changed to fat driver acceptable names in some
places.
Fat driver expects file paths to have form:
0:/path/file
while mynewt file system represents fat paths in form:
mmc0:/path/file

This conversion was done in fatfs_open, fatfs_opendir functions.

fatfs_unlinke, fatfs_rename, fatfs_mkdir were missing this
conversion.

Converting code is now extracted to function and used in
required places.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core