Several fixes to the CC2538 bsp

The pull request contains the following:
a) Added code to initialize and select the internal and external antennas via GPIOs
b) Added code to initilize the clocks appropriately (System = 32 MHz, Peripherals = 16 MHz)
c) Added code to initialize the appropriate sleep mode, right now it only goes to PM0 (WFI)
d) Added code to initialize the GPIOs for low power consumptions
e) Added code to initialize the peripherals according to the sleep mode
f) Fixed a bug in the radio that caused to read two more bytes from the receive FIFO
g) Removed a comment from the startup code to ensure that it works with the J-Link
h) Fixed the UART driver to ensure that it is configured appropriately and remove a bug related to the interrupts.
All the fixes have been tested and seem to be working fine.