affects all hiby targets, fiiom3k, and ibasso dx50/dx90
As well as deduplicating a small pile of code, this also implements
hysteresis so we're not doing a sysfs read/lookup multiple times
back-to-back every time the power management tick fires.
Change-Id: I2f7672acbb36341becf67e07960c24c681270d09
* Enabled MULTIDRIVE and ROOTFS_HOTPLUG for full hotswap!
(Requires updated bootloader/integration!)
Change-Id: I7dad1f8d375e087c36fce37bec3e9184602ede50