Power button logic is input voltage dependent

[ke5gdb]

The power down logic is somewhat input voltage dependent. Using a pair of AA alkaline batteries, everything works as expected. However a single-cell LiPo battery at 3.8v was enough to erroneously trigger the shutdown logic.

The fix should be relatively straightforward: compare the VBAT value against the SW value and if an excessive voltage delta exists, then power down.

I should have some time to dig into this issue this weekend, but I wanted to document it for visibility. It's very likely this same bug affects the RS41ng code, but I'll address that over on that repo at a later date.