it is supposed that the module is just powered on and the
AT#SSLSECCFG command is entered without
<cert_format> parameter, the default format is PEM. In this case
the AT#SSLSECCFG? read command doesn't return the setting of
the format in order to meet retro compatibility with other families.
Now, let's assume that AT#SSLSECCFG command is entered
again, but using the <cert_format> parameter for the first time: if
the read command is entered, it reports the parameter value just
used. If subsequently the <cert_format> is omitted, the
AT#SSLSECCFG? read command reports the parameter value
entered the last time.