v2.0.0

This is a major update to Nerves Heart.

• Changes
  • BREAKING: the informational attribute names changed to clarify whether they
    came from the watchdog timer device driver or Nerves heart
  • Added an optional initialization handshake to protect against
    :heart.set_callback/2 not being run and an issue going undetected.
  • Added support for guarded reboot and poweroff requests. These work similar
    to the reboot and poweroff shell commands, but stop petting the watchdog
    as well. This protects against rare reboot/poweroff hangs.
  • Pet the hardware watchdog before exiting to reduce the chance of it
    rebooting the system early due to unlucky timing from the previous pet.
  • Remove hardcoded hardware watchdog pet time and calculate based on actual
    timeout value.