Add a board level config upload.gpiostring

This config can be used to specify a custom reset method during serial upload.

Example:
```
  "upload": {
    "maximum_ram_size": 65536,
    "maximum_size": 262144,
    "protocol": "serial",
    "protocols": [
      "dfu",
      "serial",
      "jlink",
      "stlink",
      "blackmagic"
    ],
    "gpiostring": "-dtr,-rts,"
```

Fix #257

Author: Miceuz

builder/main.py

@@ -267,13 +267,13 @@ def __configure_upload_port(env):
 elif upload_protocol == "serial":
     def __configure_upload_port(env):
         return env.subst("$UPLOAD_PORT")
-
     env.Replace(
         __configure_upload_port=__configure_upload_port,
         UPLOADER=join(
             '"%s"' % platform.get_package_dir("tool-stm32duino") or "",
             "stm32flash", "stm32flash"),
         UPLOADERFLAGS=[
+            "-i%s" % board.get("upload.gpiostring") if board.get("upload.gpiostring", False) else "",
             "-g", board.get("upload.offset_address", "0x08000000"),
             "-b", env.subst("$UPLOAD_SPEED") or "115200", "-w"
         ],