fix(modem): Make the escape sequence configurable

components/esp_modem/Kconfig

@@ -76,4 +76,13 @@ menu "esp-modem"
         help
             If enabled, APIs to add URC handler are available
 
+    config ESP_MODEM_PPP_ESCAPE_BEFORE_EXIT
+        bool "Send escape sequence when switching PPP -> CMD"
+        default n
+        help
+            If enabled, the library sends a PPP escape ("+++" command)
+            to switch to command mode. This make switching from PPP to CMD
+            mode more robust for some devices (e.g. Quectel), but might cause
+            trouble for other devices (e.g. SIMCOM).
+
 endmenu

components/esp_modem/src/esp_modem_dce.cpp

@@ -45,8 +45,10 @@ static bool exit_data(DTE &dte, ModuleIf &device, Netif &netif)
     });
     netif.stop();
     netif.wait_until_ppp_exits();
+#ifdef ESP_MODEM_PPP_ESCAPE_BEFORE_EXIT
     std::array<uint8_t, 3> ppp_escape = {'+', '+', '+'};
     dte.write(ppp_escape.data(), ppp_escape.size());
+#endif
     if (!signal->wait(1, 2000)) { // wait for any of the disconnection messages
         // if no reply -> set device to command mode
         dte.set_read_cb(nullptr);