drivers/at: add a function read a response plus the OK

There are many modem commands for which you get a line of response followed
by an OK. Take for example the AT+CGSN command to get the IMEI of a Ublox
G350.
  >> AT+CGSN
  << 004999010640000
  << OK

drivers/at/at.c

@@ -228,6 +228,56 @@ out:
     return res;
 }
 
+ssize_t at_send_cmd_get_resp_wait_ok(at_dev_t *dev, const char *command, const char *resp_prefix,
+                                     char *resp_buf, size_t len, uint32_t timeout)
+{
+    ssize_t res;
+    ssize_t res_ok;
+    char ok_buf[64];
+
+    at_drain(dev);
+
+    res = at_send_cmd(dev, command, timeout);
+    if (res) {
+        goto out;
+    }
+
+    res = at_readline(dev, resp_buf, len, false, timeout);
+    if (res == 0) {
+        /* skip possible empty line */
+        res = at_readline(dev, resp_buf, len, false, timeout);
+    }
+
+    /* Strip the expected prefix */
+    if (res > 0 && resp_prefix && *resp_prefix) {
+        size_t prefix_len = strlen(resp_prefix);
+        if (strncmp(resp_buf, resp_prefix, prefix_len) == 0) {
+            size_t remaining_len = strlen(resp_buf) - prefix_len;
+            /* The one extra byte in the copy is the terminating nul byte */
+            memmove(resp_buf, resp_buf + prefix_len, remaining_len + 1);
+            res -= prefix_len;
+        }
+    }
+
+    /* wait for OK */
+    if (res >= 0) {
+        res_ok = at_readline(dev, ok_buf, sizeof(ok_buf), false, timeout);
+        if (res_ok == 0) {
+            /* skip possible empty line */
+            res_ok = at_readline(dev, ok_buf, sizeof(ok_buf), false, timeout);
+        }
+        ssize_t len_ok = sizeof(CONFIG_AT_RECV_OK) - 1;
+        if ((len_ok != 0) && (strcmp(ok_buf, CONFIG_AT_RECV_OK) == 0)) {
+        }
+        else {
+            /* Something else then OK */
+            res = -1;
+        }
+    }
+out:
+    return res;
+}
+
 ssize_t at_send_cmd_get_lines(at_dev_t *dev, const char *command,
                               char *resp_buf, size_t len, bool keep_eol, uint32_t timeout)
 {

drivers/include/at.h

@@ -248,6 +248,27 @@ int at_send_cmd_wait_prompt(at_dev_t *dev, const char *command, uint32_t timeout
  */
 ssize_t at_send_cmd_get_resp(at_dev_t *dev, const char *command, char *resp_buf, size_t len, uint32_t timeout);
 
+/**
+ * @brief   Send AT command, wait for response plus OK
+ *
+ * This function sends the supplied @p command, then waits and returns one
+ * line of response.
+ *
+ * A possible empty line will be skipped.
+ *
+ * @param[in]   dev         device to operate on
+ * @param[in]   command     command to send
+ * @param[in]   resp_prefix expected prefix in the response
+ * @param[out]  resp_buf    buffer for storing response
+ * @param[in]   len         len of @p buffer
+ * @param[in]   timeout     timeout (in usec)
+ *
+ * @returns     length of response on success
+ * @returns     <0 on error
+ */
+ssize_t at_send_cmd_get_resp_wait_ok(at_dev_t *dev, const char *command, const char *resp_prefix,
+                                     char *resp_buf, size_t len, uint32_t timeout);
+
 /**
  * @brief   Send AT command, wait for multiline response
  *