Add L2 PPP and PPPoE

src/drivers/at_cmd.h

@@ -0,0 +1,14 @@
+#pragma once
+
+struct mg_tcpip_driver_atcmd_data {
+  void *usart;                  // Opaque UART/SPI descriptor
+  void (*reset)(void *);        // Modem hardware reset
+  void (*tx)(void *, uint8_t);  // USART transmit single byte
+  int (*rx)(void *);            // USART receive single byte
+  const char **script;          // List of AT commands and expected replies
+  int script_index;             // Index of the current AT command in the list
+  uint64_t deadline;            // AT command deadline in ms
+  bool no_byte_stuff;           // Do not perform byte stuffing/unstuffing
+  bool link;                    // internal use: done with AT processing
+  bool unstuffing;              // internal use: unstuffing state
+};