wifi: eswifi: Add controller at command helpers

Create eswifi_at_cmd and eswifi_at_cmd_rsp helpers, allowing to send an
at command and parse the at response. These methods return success if
the response contains the OK* string. The eswifi_at_cmd_rsp method
extracts response content/size (DATA) on success.

*Response format is:
\r\n[DATA]\r\nOK\r\n>
Where DATA can be arbitrary (ASCII or not).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>