drivers: wifi: simplelink: Implement setsockopt() for TLS offload

Implements setsockopt() for the socket offload driver
to process the TLS tags sent in via the Zephyr setsockopt() API,
when CONFIG_NET_SOCKETS_SOCKOPT_TLS is chosen.
For each tag, the credential filenames are retrieved and
set via SimpleLink's sl_SetSockOpt() API.

Also, creates a new KConfig option for TLS_CREDENTIAL_FILENAMES.
This new option is used by apps/protocols to add TLS credentials
via filenames referring to the actual content stored on a secure
file system or flash.

Handles the IPPROTO_TLS_* socket protocol families in the
socket() offloaded API.

This was validated on the cc3220sf_launchxl with the http_get sockets
sample, with the globalsign_r2.der file loaded to secure flash via
the TI Uniflash tool, and using the TI Catalog of known good
root CA's.

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>