new standalone drivers/system_power (split out of adc)

- system_power portion of drivers/adc split into a new standalone module (drivers/system_power)
 - additional availability flags added to system_power.msg so we know what boards even support (previously certain flags were faked if not available)
 - drivers/system_power scheduled at minimum rate by default, but now also scheduled immediately on any relevant GPIO events
 - now instead of running at 100 Hz with heavily reduced logging we can run at a minimal rate, log at full rate, and catch relevant changes immediately

boards/uvify/core/init/rc.board_sensors

@@ -4,6 +4,7 @@
 #------------------------------------------------------------------------------
 
 board_adc start
+system_power start
 
 # Internal SPI
 if ! ms5611 -T 5607 -s start