The current code does hazardous tweaks to the power subsystem: indeed if one
boots with USB plugged and some stub powers on the DCDC switch, it will fail.
Indeed, a hardware bug prevents from going back to linear regulators (see
errata) so we cannot expect to reach a known state (linreg on, dcdc off)
on each configuration and in particular, powering down the 4p2 rail in
such a configuration will result in a power brownout.
This commit works around this issue by not touching the initial power
configuration until USB is (un)plugged, which are the best spots to get
known states.
Change-Id: I8741a3995df8ae61ca1c887a3ecb7903d0ac5136
c043ecf9e1