DataFlash: common Log_Write_Camera

pair programmed with Craig Elder
Copter: use common Log_Write_Camera

pair programmed with Craig Elder
Copter: flag current alt as relative

This is required so common Log_Write_Camera can calculate relative and
abs altitudes provided by either copter or plane

pair programmed with Craig Elder
Copter: bug fix to remove home variable

pair programmed with Craig Elder
Rover: use common Log_Write_Camera

pair programmed with Craig Elder
Plane: use common Log_Write_Camera

pair programmed with Craig Elder