platform-nordicnrf51 is missing the generic.json nRF51 variant for Generic_nRF51822

[zpm1066]

The nRF51 variant json file for Generic_nRF51822 is missing in /.platformio/platforms/nordicnrf51/boards.

I have verified that the following generic.json works fine with nRF51822 (and nRF51802) boards.

{
  "build": {
    "arduino":{
      "ldscript": "nrf51_xxaa.ld"
    },
    "core": "nRF5",
    "cpu": "cortex-m0",
    "extra_flags": "-DARDUINO_BBC_MICROBIT -DUSE_LFRC",
    "f_cpu": "16000000L",
    "mcu": "nrf51822",
    "variant": "Generic",
    "zephyr": {
       "variant": "generic_nrf51"
    }
  },
  "connectivity": [
    "bluetooth"
  ],
  "debug": {
    "onboard_tools": [
      "jlink"
    ],
    "svd_path": "nrf51.svd",
    "jlink_device": "nRF51822_xxAA"
  },
  "frameworks": [
    "arduino",
    "mbed",
    "zephyr"
  ],
  "name": "Generic nRF51",
  "upload": {
    "maximum_ram_size": 16384,
    "maximum_size": 262144,
    "protocol": "jlink",
    "protocols": [
      "cmsis-dap",
      "mbed",
      "jlink"
    ]
  },
  "url": "https://www.nordicsemi.com/Products/nRF51822",
  "vendor": "Nordic"
}

Please review and update the repo.