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
