ESP8266: Rework the execution flow to match the SAMD21.

* Support code.txt and variants.
* Add wait state for connection indicated by receiving a character.
* Add another wait state on every soft reset to give instruction for
  soft reset rather than entering REPL.

AMPY use remains the same. The wait states can be exited with CTRL-C
just like the REPL.

Fixes #70.

esp8266/main.c

@@ -34,6 +34,7 @@
 #include "py/stackctrl.h"
 #include "py/mphal.h"
 #include "py/gc.h"
+#include "lib/fatfs/ff.h"
 #include "lib/mp-readline/readline.h"
 #include "lib/utils/pyexec.h"
 #include "gccollect.h"
@@ -43,6 +44,54 @@
 
 STATIC char heap[36 * 1024];
 
+bool maybe_run(const char* filename, pyexec_result_t* exec_result) {
+    FILINFO fno;
+#if _USE_LFN
+    fno.lfname = NULL;
+    fno.lfsize = 0;
+#endif
+    FRESULT res = f_stat(filename, &fno);
+    if (res != FR_OK || fno.fattrib & AM_DIR) {
+        return false;
+    }
+    mp_hal_stdout_tx_str(filename);
+    mp_hal_stdout_tx_str(" output:\r\n");
+    pyexec_file(filename, exec_result);
+    return true;
+}
+
+bool serial_active = false;
+
+STATIC bool start_mp(void) {
+    pyexec_frozen_module("_boot.py");
+
+    pyexec_result_t result;
+    bool found_boot = maybe_run("settings.txt", &result) ||
+                      maybe_run("settings.py", &result) ||
+                      maybe_run("boot.py", &result) ||
+                      maybe_run("boot.txt", &result);
+
+    if (!found_boot || !(result.return_code & PYEXEC_FORCED_EXIT)) {
+        maybe_run("code.txt", &result) ||
+            maybe_run("code.py", &result) ||
+            maybe_run("main.py", &result) ||
+            maybe_run("main.txt", &result);
+    }
+
+    if (result.return_code & PYEXEC_FORCED_EXIT) {
+        return false;
+    }
+
+    // We can't detect connections so we wait for any character to mark the serial active.
+    if (!serial_active) {
+        mp_hal_stdin_rx_chr();
+        serial_active = true;
+    }
+    mp_hal_stdout_tx_str("\r\n\r\n");
+    mp_hal_stdout_tx_str("Press any key to enter the REPL. Use CTRL-D to soft reset.\r\n");
+    return mp_hal_stdin_rx_chr() == CHAR_CTRL_D;
+}
+
 STATIC void mp_reset(void) {
     mp_stack_set_top((void*)0x40000000);
     mp_stack_set_limit(8192);
@@ -65,48 +114,37 @@ STATIC void mp_reset(void) {
     readline_init0();
     dupterm_task_init();
     pwmout_reset();
-#if MICROPY_MODULE_FROZEN
-    pyexec_frozen_module("_boot.py");
-    pyexec_file("boot.py", NULL);
-    pyexec_file("main.py", NULL);
-#endif
 }
 
-void soft_reset(void) {
+bool soft_reset(void) {
     mp_hal_stdout_tx_str("PYB: soft reboot\r\n");
     mp_hal_delay_us(10000); // allow UART to flush output
     mp_reset();
-    #if MICROPY_REPL_EVENT_DRIVEN
+    mp_hal_delay_us(1000); // Give the RTOS time to do housekeeping.
-    pyexec_event_repl_init();
+    return start_mp();
-    #endif
 }
 
 void init_done(void) {
-    #if MICROPY_REPL_EVENT_DRIVEN
-    uart_task_init();
-    #endif
     mp_reset();
+    mp_hal_delay_us(1000); // Give the RTOS time to do housekeeping.
+    bool skip_repl = start_mp();
     mp_hal_stdout_tx_str("\r\n");
-    #if MICROPY_REPL_EVENT_DRIVEN
-    pyexec_event_repl_init();
-    #endif
 
-    #if !MICROPY_REPL_EVENT_DRIVEN
+    int exit_code = PYEXEC_FORCED_EXIT;
-soft_reset:
+    while (true) {
-    for (;;) {
+        if (!skip_repl) {
-        if (pyexec_mode_kind == PYEXEC_MODE_RAW_REPL) {
+            if (pyexec_mode_kind == PYEXEC_MODE_RAW_REPL) {
-            if (pyexec_raw_repl() != 0) {
+                exit_code = pyexec_raw_repl();
-                break;
+            } else {
-            }
+                exit_code = pyexec_friendly_repl();
-        } else {
-            if (pyexec_friendly_repl() != 0) {
-                break;
             }
         }
+        if (exit_code == PYEXEC_FORCED_EXIT) {
+            skip_repl = soft_reset();
+        } else if (exit_code != 0) {
+            break;
+        }
     }
-    soft_reset();
-    goto soft_reset;
-    #endif
 }
 
 void user_init(void) {