Replace qfn-28_4x4_P0.5 with scripted version

Script is now capable of reducing the heel fillet for only corner pins (used here in addition with chamfering it to achieve the required clearance for these pads.)

Datasheet: http://www.st.com/resource/en/datasheet/stm32f031k6.pdf Script changes: https://github.com/pointhi/kicad-footprint-generator/pull/149

Not build exactly after the datasheet but created using IPC parameters. (larger heel and toe fillets for normal pins. a bit smaller side fillet) As this is not a manufacturer specific footprint but a generic one this should be ok.

Parameters:

QFN-28_4x4mm_P0.5mm:
  device_type: 'QFN'
  size_source: 'http://www.st.com/resource/en/datasheet/stm32f031k6.pdf (Page 280)'
  ipc_class: 'qfn' # 'qfn_pull_back'
  body_size_x:
    nominal: 4
    tolerance: 0
  body_size_y:
    nominal: 4
    tolerance: 0
  lead_width:
    minimum: 0.17
    maximum: 0.25
  lead_len:
    minimum: 0.3
    maximum: 0.5

  pitch: 0.5
  num_pins_x: 7
  num_pins_y: 7
  chamfer_edge_pins: 0.15
  edge_heel_reduction: 0.1