Make the CC2538 UART driver more configurable

`platform/cc2538dk/dev/board.h` included defines for `UART_RX_PORT`, `UART_RX_PIN` and so on, suggesting that it was possible to configure the CC2538 UART driver with custom port/pin assignments for RX/TX etc through defines.

It wasn't possible. Now it is.

This pull request allows us to use defines to:
* Easily configure a cc2538-based platform to switch between UART0 and UART1.
* Configure the UART RX and TX port/pin assignment