The CC3120 Programmers guide states (page 117):
"The service pack is a special file which already contains the signature. When writing the
service pack by the host, the sl_FsClose() function should receive a NULL certificate name
and a NULL signature."
But signature file still exists in the "servicepack-cc3x20" folder of the SDK.
I've tried to write service pack file and then closed it with NULL signature, but got SL_ERROR_FS_ILLEGAL_SIGNATURE. When I use the signature provided with SDK, there is no problem: sl_FsClose returns 0.
So, maybe there is just a mistake in the doc?