Radio Config: add centered spinner on launch

- Update spinner in firmware update manager to
  use [GCSActivityIndicator centerOnView: method]
Radio Config: remove spinner once radio settinigs load

- Show UIAlert view if no FWR accessory attached when
  config radio button is touched.

- Enter radio config mode as view appears with Save
  button disabled

- Enable Save button once existing radio settings
  are loaded and spinner removed.
Merge pull request #90 from fightingwalrus/igcs-151-radio-config-spinner

Igcs 151 radio config spinner