net: l2: wifi: Make build time certs as optional

Certificates usage depends on STA/AP mode, but we don't have that
information at a build time, so, make all certs as optional and if a
file isn't found then generate an empty header so that corresponding C
code will be built.

Any missing mandatory certificates will be validated before connection
and connection is failed.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>