* energest time defaults to rtimer but can be configured via
  ENERGEST_CONF_CURRENT_TIME and ENERGEST_CONF_SECOND.

* Additional energest types can be added by platform and application
  using ENERGEST_CONF_PLATFORM_ADDITIONS and ENERGEST_CONF_ADDITIONS.

* Energest times are now 64 bit and thereby non-wrapping in most systems.

* Removed unused energest types for serial and sensors.