fix(modem): Fix netif data race causing PPP startup delays

Removes PPP_started signal on reception, since lwip handles data
reception if the netif is up/down (which we correctly set in start()
stop() methods)

Closes #308

components/esp_modem/include/cxx_include/esp_modem_netif.hpp

@@ -54,9 +54,10 @@ class Netif {
      */
     void stop();
 
-private:
     void receive(uint8_t *data, size_t len);
 
+private:
+
     static esp_err_t esp_modem_dte_transmit(void *h, void *buffer, size_t len);
 
     static esp_err_t esp_modem_post_attach(esp_netif_t *esp_netif, void *args);

components/esp_modem/src/esp_modem_dce.cpp

@@ -21,10 +21,17 @@ static bool exit_data(DTE &dte, ModuleIf &device, Netif &netif)
     netif.stop();
     auto signal = std::make_shared<SignalGroup>();
     std::weak_ptr<SignalGroup> weak_signal = signal;
-    dte.set_read_cb([weak_signal](uint8_t *data, size_t len) -> bool {
+    dte.set_read_cb([&netif, weak_signal](uint8_t *data, size_t len) -> bool {
+        // post the transitioning data to the network layers if it contains PPP SOF marker
+        if (memchr(data, 0x7E, len))
+        {
+            ESP_LOG_BUFFER_HEXDUMP("esp-modem: debug_data (PPP)", data, len, ESP_LOG_DEBUG);
+            netif.receive(data, len);
+        }
+        // treat the transitioning data as a textual message if it contains a newline char
         if (memchr(data, '\n', len))
         {
-            ESP_LOG_BUFFER_HEXDUMP("esp-modem: debug_data", data, len, ESP_LOG_DEBUG);
+            ESP_LOG_BUFFER_HEXDUMP("esp-modem: debug_data (CMD)", data, len, ESP_LOG_DEBUG);
             const auto pass = std::list<std::string_view>({"NO CARRIER", "DISCONNECTED"});
             std::string_view response((char *) data, len);
             for (auto &it : pass)

components/esp_modem/src/esp_modem_netif.cpp

@@ -65,9 +65,7 @@ esp_err_t Netif::esp_modem_post_attach(esp_netif_t *esp_netif, void *args)
 
 void Netif::receive(uint8_t *data, size_t len)
 {
-    if (signal.is_any(PPP_STARTED)) {
-        esp_netif_receive(driver.base.netif, data, len, nullptr);
-    }
+    esp_netif_receive(driver.base.netif, data, len, nullptr);
 }
 
 Netif::Netif(std::shared_ptr<DTE> e, esp_netif_t *ppp_netif) :
@@ -89,8 +87,8 @@ void Netif::start()
         receive(data, len);
         return true;
     });
-    esp_netif_action_start(driver.base.netif, nullptr, 0, nullptr);
     signal.set(PPP_STARTED);
+    esp_netif_action_start(driver.base.netif, nullptr, 0, nullptr);
 }
 
 void Netif::stop()