Have CBM DOS filetype of imported files match the one used by cc65.

CFS_WRITE implies O_TRUNC which is implemented on CBM DOS by deleting an
exsisting file. Hoewever this succeeds only if the CBM DOS filetype matches.

We need a working O_TRUNC in order to be able to overwrite the contiki.cfg
configuration file.

Note: Now it has be clarified why overwriting the configuration file started to
fail the CBM PFS (platform file system) can be activated for the recently added
ethconfig program.