UART0 pin as backdoor enable pin on CC26xx and CC13xx

[JelmerT]

Found this interesting line in the Tech Ref Manual of the CC1350

If the backdoor pin configuration matches one of the UART0 or SSI0 pins, the external user must deassert the backdoor signal before transmitting on the UART0 or SSI0 interface.

Pretty sure this wasn't possible with the CC2538, but probably it is with the CC26xx. So if we use the RX line for the bootloader enable pin, we could make entering the bootloader slightly easier. Instead of having to do the select + reset finger dance, you'd just need to reset the chip.