energest: made energest more configurable
* 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.