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
The Q and K have a slightly different case, but the hardware under the
shell is completely identical.
These models are rebadged versions:
* Hifiwalker H2 (== Q)
* AGPTek H3 (== K)
* Surfans F20 (== K)
Other notes:
* Significant improvements in the shared Hiby-platform launcher/loader
* SD card can theoretically be hot-swapped now
* Support external USB mass storage!
* Some consolidation of Hiby-platform targets
* Some consolidation of plugin keymaps
Todo/known issues:
* Keymaps need to be gone over properly
* Convert to HAVE_SCROLLWHEEL?
Change-Id: I5a8a4f22c38a5b69392ca7c0a8ad8c4e07d9523c
