Added board power button notification API
Added board power button notification API

   Defined types and interface to support a notification call back
   on power button events.

mje-nz/PX4-Firmware,jlecoeur/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,jlecoeur/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,acfloria/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,PX4/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,acfloria/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,krbeverx/Firmware,krbeverx/Firmware,Aerotenna/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware