ToneAlarm class refactoring to implement an interface for hardware specific methods and a single ToneAlarm class.

instead of the derivative of the vertical position which is
not the same value when using the ekf2 estimation.

Using the position derivative resulted in delayed landing because
the value for some reason often bumped over the theshold before
slowly converging towards zero while the velocity was within expected
accuary in all these cases.

    - micro-CDR in PX4/Firmware (e0680ffe2280620b81d415328f109dc8decaa1b8): https://github.com/eProsima/micro-CDR/commit/7a76880f90dfe702f4fa1723e22d1dcb491d7b9e
    - micro-CDR current upstream: https://github.com/eProsima/micro-CDR/commit/62d95c870eafb0cccc7bf70bb8a0cbb86f125a0e
    - Changes: https://github.com/eProsima/micro-CDR/compare/7a76880f90dfe702f4fa1723e22d1dcb491d7b9e...62d95c870eafb0cccc7bf70bb8a0cbb86f125a0e

    62d95c8 2019-01-24 Luis Enrique Muñoz Martín - Refs #4438. Modified the configuration of machine endianness. (#31)

* mavlink_receiver: generate rc channel count
* mavlink_receiver: zero out ignored rc channels, add comments

 - needed if NuttX networking enabled

   Upstream header file is conditional on CONFIG_ARCH_HAVE_PROGMEM

   Suport upstream change in nuttx.

   Upstream changes defer the board reset based on the vaule of
   CONFIG_BOARD_RESET_ON_ASSERT
      0 (or not defined) Do not reset on assert
      >= 1 reset if assertion is in an interrupt handler or the idle task
      >= 2 on any asertion.

  up_assert is called from up_hardfault or an asertion failure.
  Part 1 of up_assert will call out to the board_crashdump
  Part 2 on return from board_crashdump will then perform
  the reset.

  board_crashdump needs a chunk of ram to save the complete
  context in. It uses  &_sdata which is the lowest memory and
  it will corrupt that memeory.  We can therfore can not allow
  return to the OS, as it could depend on that area of RAM.

  So all boards need to do a reset at the end of board_crashdump

   The F1 series GTIMs to not have GTIM_CCER_CC1NP.
   This bug fix tracks the upstream change that
   made GTIM_CCER_CC1NP conditional on the chip.

system_state.fmu_data_received_time can be set from an IRQ handler, thus
we need to ensure every read access to it in mixer_tick is atomic.
So we read it once and copy it into a local variable.

GCC did not do that.

This parameter was removed in d704a2d6 and should
not be referenced anymore.