Wire nRPIBOOT to a button, figure out how to use nRPIBOOT as GPIO

[EvanKrall]

I'm pretty sure I saw something that suggested that nRPIBOOT is just a regular GPIO internally, so we can kill two birds with one stone by wiring nRPIBOOT to a button (maybe volume down?)

• have a button that allows us to force rpiboot mode
• free up a GPIO for other uses (e.g. max16150 times out before nEXTRST goes high #24)

[EvanKrall]

https://www.alexwhittemore.com/raspberry-pi-cm4-using-nrpi_boot/