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

1. only port the uart0 driver.
2. use INT vector table in SRAM (only work with keil too chain).
3. compile test with GCC (Sourcery CodeBench Lite 2014.05-28) tool chains
4. compile test with Keil and IAR tool chains
5. modified the keil template to generate MDK project file from scons.
6. modified the iar template to generate ewarm workspace/project files from scons.