Add a board level config `upload.gpiostring`

[Miceuz]

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

Also fix a small bug where 'False' would be inserted in case when tool-stm32duino package is not available.