[bsp] Add BSP for TM4C1294XL (TI Tiva C Series Connected LaunchPad)

1. only port the uart0 driver.
2. use INT vector table in SRAM.
3. compile test with GCC (Sourcery CodeBench Lite 2014.05-28) and Keil (MDK 5.11.0.0)tool chains.
4. modified the keil template to generate MDK project file from scons.