lwip: add compatibility headers for sntp.h and esp_ping.h

Before LwIP was moved into a submodule, sntp and esp_ping were
available as the following includes:

#include "apps/sntp/sntp.h"
#include "esp_ping.h"

This commit makes header files available at the old location, and adds
a deprecation warning for sntp.h.

Closes https://github.com/espressif/esp-idf/issues/2392

components/esp32/esp_err_to_name.c

@@ -4,8 +4,8 @@
 #if __has_include("soc/soc.h")
 #include "soc/soc.h"
 #endif
-#if __has_include("apps/ping/esp_ping.h")
-#include "apps/ping/esp_ping.h"
+#if __has_include("apps/esp_ping.h")
+#include "apps/esp_ping.h"
 #endif
 #if __has_include("esp32/ulp.h")
 #include "esp32/ulp.h"
@@ -399,7 +399,7 @@ static const esp_err_msg_t esp_err_msg_table[] = {
 #   ifdef      ESP_ERR_TCPIP_ADAPTER_DHCP_NOT_STOPPED
     ERR_TBL_IT(ESP_ERR_TCPIP_ADAPTER_DHCP_NOT_STOPPED),     /* 20487 0x5007 */
 #   endif
-    // components/lwip/include/apps/ping/esp_ping.h
+    // components/lwip/include/apps/esp_ping.h
 #   ifdef      ESP_ERR_PING_BASE
     ERR_TBL_IT(ESP_ERR_PING_BASE),                          /* 24576 0x6000 */
 #   endif

components/lwip/CMakeLists.txt

@@ -3,6 +3,7 @@ set(COMPONENT_ADD_INCLUDEDIRS
     lwip/src/include
     port/esp32/include
     port/esp32/include/arch
+    include_compat
     )
 
 set(COMPONENT_SRCS "apps/dhcpserver/dhcpserver.c"

components/lwip/apps/ping/esp_ping.c

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 #include <string.h>
-#include "ping/esp_ping.h"
+#include "esp_ping.h"
 
 #include "lwip/ip_addr.h"
 

components/lwip/apps/ping/ping.c

@@ -58,7 +58,7 @@
 #endif /* PING_USE_SOCKETS */
 
 #ifdef ESP_PING
-#include "ping/esp_ping.h"
+#include "esp_ping.h"
 #include "lwip/ip_addr.h"
 #endif
 /**

components/lwip/component.mk

@@ -7,7 +7,8 @@ COMPONENT_ADD_INCLUDEDIRS := \
 	include/apps \
 	lwip/src/include \
 	port/esp32/include \
-	port/esp32/include/arch
+	port/esp32/include/arch \
+	include_compat
 
 COMPONENT_SRCDIRS := \
 	apps/dhcpserver \

components/lwip/include_compat/apps/sntp/sntp.h

@@ -0,0 +1,3 @@
+#pragma once
+#warning "This header file is deprecated, please include lwip/apps/sntp.h instead."
+#include "lwip/apps/sntp.h"