net: l2: wifi: ensure certificates directory is created

Ensure that the output certificates directory is created, where
generated certificates will be placed. This fixes a build error seen
when using `make` to build samples/net/wifi for the rd_rw612_bga board,
where the output directory for generated certificates did not exist at
build time.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>