Enable OTA Update mechanism for any mDNS enabled platform

[me-no-dev]

Adds ability to update the firmware of non-SSH boards using TXT
parameters from mDNS

Three new TXT keys are added (defaults are capital):

1. ssh_upload=YES/no
2. tcp_check=YES/no
3. auth_upload=yes/NO

• "ssh_upload" controls which Uploader should be used. Defaults to
  SSHUploader
• "tcp_check" controls wether TCP reachability test should be performed.
  Defaults to "yes". Boards that have few TCP ports available, can use
  "no" to skip the check and update over UDP (ESP8266 for example).
• "auth_upload" controls wether authentication is required (only for
  "ssh_upload=no").

One new pattern and two new variables are available.

1. Var "network.port" resolves to the advertised by mDNS device port.
2. Var "network.password" resolves to the entered device password if
   upload authentication is used, else it's empty string.
3. Pattern "upload.network_pattern" is usable if the platform is using
   different tool/command to do network updates. If not defined,
   "upload.pattern" will be used instead.

Changes are inspired by the need of the ESP8266 Arduino project to utilize it's wireless for simple updates, it does not interfere with any other previous configuration or official Arduino boards and can be applied to basically any networked MCU.