Interworking: Allow cred blocks not to be saved to a file

Interworking: Allow cred blocks not to be saved to a file

The new cred block parameter 'temporary' can be used to indicate that a
cred block is not to be saved to wpa_supplicant configuration file
(e.g., "SET_CRED 0 temporary 1"). This is similar to the concept of
temporary network blocks and allows cred blocks to be managed outside
the wpa_supplicant config file when other parameters are still saved to
the file written by wpa_supplicant.

Signed-hostap: Jouni Malinen <j@w1.fi>