FW-258. Fix.

(Finally) Fixed FW-258.
a) Removed the initialization delay of the UART peripheral since it is no longer required.
b) The UART peripheral of the CC2538 has a shadow register that needs to be emptied by a timeout interrupt, otherwise the last byte does not get processed until a new byte arrives. This affects the communication with OpenVisualizer since it relies on communicating full HDLC frames.