Added my smtp client using the lwIP raw API

apps/smtp/smtp.h

@@ -0,0 +1,67 @@
+#ifndef __SMTP_H__
+#define __SMTP_H__
+
+#include "lwip/err.h"
+
+/** The default TCP port used for SMTP */
+#define SMTP_DEFAULT_PORT  25
+
+/** Email successfully sent */
+#define SMTP_RESULT_OK            0
+/** Unknown error */
+#define SMTP_RESULT_ERR_UNKNOWN   1
+/** Connection to server failed */
+#define SMTP_RESULT_ERR_CONNECT   2
+/** Failed to resolve server hostname */
+#define SMTP_RESULT_ERR_HOSTNAME  3
+/** Connection unexpectedly closed by remote server */
+#define SMTP_RESULT_ERR_CLOSED    4
+/** Connection timed out (server didn't respond in time) */
+#define SMTP_RESULT_ERR_TIMEOUT   5
+/** Server responded with an unknown response code */
+#define SMTP_RESULT_ERR_SVR_RESP  6
+
+/** Prototype of an smtp callback function
+ *
+ * @param arg argument specified when initiating the email
+ * @param smtp_result result of the mail transfer (see defines SMTP_RESULT_*)
+ * @param srv_err if aborted by the server, this contains the error code received
+ * @param err an error returned by internal lwip functions, can help to specify
+ *            the source of the error but must not necessarily be != ERR_OK
+ */
+typedef void (*smtp_result_fn)(void *arg, u8_t smtp_result, u16_t srv_err, err_t err);
+
+/** This structure is used as argument for smtp_send_mail_int(),
+ * which in turn can be used with tcpip_callback() to send mail
+ * from interrupt context.
+ * For member description, see parameter description of smtp_send_mail().
+ * When using with tcpip_callback, this structure has to stay allocated
+ * (e.g. using mem_malloc/mem_free) until its 'callback_fn' is called.
+ */
+struct smtp_send_request {
+  const char *from;
+  const char* to;
+  const char* subject;
+  const char* body;
+  smtp_result_fn callback_fn;
+  void* callback_arg;
+  /** If this is != 0, data is *not* copied into an extra buffer
+   * but used from the pointers supplied in this struct.
+   * This means less memory usage, but data must stay untouched until
+   * the callback function is called. */
+  u8_t static_data;
+};
+
+void smtp_set_server_addr(const char* server);
+void smtp_set_server_port(u16_t port);
+err_t smtp_set_auth(const char* username, const char* pass);
+err_t smtp_send_mail(const char *from, const char* to, const char* subject, const char* body,
+                     smtp_result_fn callback_fn, void* callback_arg);
+err_t smtp_send_mail_static(const char *from, const char* to, const char* subject, const char* body,
+                     smtp_result_fn callback_fn, void* callback_arg);
+void smtp_send_mail_int(void *arg);
+#if LWIP_DEBUG
+const char* smtp_result_str(u8_t smtp_result);
+#endif
+
+#endif /* __SMTP_H__ */