[rotorcraft] v_adapt: config improvements and refactor

- GUIDANCE_V_ADAPT_INITIAL_HOVER_THROTTLE settable in % with default of 0.3
- GUIDANCE_V_ADAPT_THROTTLE_ENABLED:
  - TRUE by default if GUIDANCE_V_NOMINAL_HOVER_THROTTLE was not defined
  - FALSE by default if only GUIDANCE_V_NOMINAL_HOVER_THROTTLE was set in airframe file
    (for backwards compatibily)
- switching between nominal_trottle and the adaptive estimate during operation via settings
[conf] update some airframe examples to GUIDANCE_V_ADAPT changes