Changed the style of the UART configuration for different boards,
from a define based configuration to one based on an array of
structs, one struct for each UART, with the format of the struct defined
in cc2538/include/periph_cpu.h.

  - Defined the fields of the struct in periph_cpu.h
  - Removed the compilation includes that were in uart.c for each UART
  - Implemented a generic ISR subroutine for clarity
  - combined uart_base and uart_init in uart.c
  - used bitmask for the interrupt setup
  - took the uart Rx, Tx, and IRQ numbers out of the config
  (as this has to match the .dev field). Replaced with
  macros from the uart number
  - took out some unused code
  - implemented power on/off commands
  - removed reset function - now bytes are just discarded on error
  - Rx now not initialised if Rx callback = NULL, as per
  drivers/periph/uart.h
  - device is now enabled after callbacks are set, not before
  - asserts raised if rts and cts are enabled for UART0
  - BIT macro removed