mgmt: mcumgr: Add fs_mgmt log output workaround on file write

If a file write was attempted with a file that does not exist, over
the shell/UART when logging was enabled, it would output a fs_unlink
error, this works around the issue by checking if the file exists and
needs truncating before performing that action. It also imrproves
flash endurance slightly by performing a truncate operation instead
of a delete, but will fall back to a delete if the truncation
operation fails. This issue can be also be mitigated by altering
logging settings or adjusting the SMP thread priority.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>