[PATCH v2] mkimage: FIT ciphering bug fixes

[patrick.oppenlander at gmail.com]

The v2 series addresses review comments from Philippe Reynes:
* Use FIT_CIPHER_NODENAME instead of hard coding "cipher"
* Simplify handling of FDT_ERR_NOSPACE
* Simplify detection of previously ciphered data

The last two points are possible as I overlooked that the retry loop
handling ENOSPC in fit_handle_file() reloads the original FIT before
retrying.