Revise low voltage management

[jbwillis]

If VBAT_OK goes low while the radio (highest power device) is on, the MCU will get turned off and the radio will stay on.

Two options:

• allow the mcu to do all power management - VBAT_OK could be fed to the MCU
• use VBAT_OK to disconnect all devices from the batteries - this entails more than just disabling 3V3 since the radio and payload not on the 3V3 circuit. Perhaps we skip the harvester regulator and put a buck-boost converter on the mainboard for the MCU.

[jbwillis]

Use the 3V3 line as a signal to turn off the high power line