tee-supplicant: Synchronize file operations for secure storage

In order to properly synchronize data and meta-data for secure
storage, fsync() must be performed on the containing directory
after mkdir() and rename() operations.
This may be especially of interest in a production environment
where power may be cut as soon as all the security
parameters are saved.

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Joakim Nordell <joakim.nordell@axis.com>