tools/mpremote: Improve reliability of mount after soft reboot.

With the existing code problems can occur with remounting, the "if t -
t_last_activity > QUIET_TIMEOUT:" check can be triggered early before the
REPL string comes through, meaning that the remount doesn't happen.

On certain boards the "MPY: soft reboot" line comes through immediately
(getting the routine past initial timeout) but then there's a slightly
longer delay while the board restarts before it prints out the startup
header and the REPL prompt.

This commit adds some extra pattern monitoring during the timeout loop to
track the state if a soft restart is actually started.

tools/mpremote/mpremote/pyboardextended.py

@@ -628,10 +628,13 @@ def write_ctrl_d(self, out_callback):
 
         # Read response from the device until it is quiet (with a timeout).
         INITIAL_TIMEOUT = 0.5
-        QUIET_TIMEOUT = 0.2
+        BANNER_TIMEOUT = 2
+        QUIET_TIMEOUT = 0.1
         FULL_TIMEOUT = 5
         t_start = t_last_activity = time.monotonic()
         data_all = b""
+        soft_reboot_started = False
+        soft_reboot_banner = False
         while True:
             t = time.monotonic()
             n = self.serial.inWaiting()
@@ -646,13 +649,36 @@ def write_ctrl_d(self, out_callback):
                         return
                 else:
                     if t - t_start > FULL_TIMEOUT:
+                        if soft_reboot_started:
+                            break
                         return
-                    if t - t_last_activity > QUIET_TIMEOUT:
+
+                    next_data_timeout = QUIET_TIMEOUT
+
+                    if not soft_reboot_started and data_all.find(b"MPY: soft reboot") != -1:
+                        soft_reboot_started = True
+
+                    if soft_reboot_started and not soft_reboot_banner:
+                        # Once soft reboot has been initiated, give some more time for the startup
+                        # banner to be shown
+                        if data_all.find(b"\nMicroPython ") != -1:
+                            soft_reboot_banner = True
+                        elif data_all.find(b"\nraw REPL; CTRL-B to exit\r\n") != -1:
+                            soft_reboot_banner = True
+                        else:
+                            next_data_timeout = BANNER_TIMEOUT
+
+                    if t - t_last_activity > next_data_timeout:
                         break
 
-        # Check if a soft reset occurred.
-        if data_all.find(b"MPY: soft reboot") == -1:
+        if not soft_reboot_started:
             return
+
+        if not soft_reboot_banner:
+            out_callback(b"Warning: Could not remount local filesystem\r\n")
+            return
+
+        # Determine type of prompt
         if data_all.endswith(b">"):
             in_friendly_repl = False
             prompt = b">"