Add TI LM4F120XL board :
 - Add TI library;
 - HAL clock for this board is provided;
 - HAL gpio for this board is provided.
Modify gpio example to run it on TI LM4F120XL board.
Add lm4f120 (partial) support in README.md and a command line example.
start of linker script refactoring

use the same script for stm32f100 and stm32f103 => it seems to be possible to use the same script for all board.
add an option to use lto

cmake  ../../../embedded/dev/board_examples/uart/ -DCMAKE_BUILD_TYPE=Release -Darch=stm32f103 -DUSE_LTO=on
TI LM4F120XL clock improvements :
- Set board system clock to 80 Mhz (16 Mhz previously).
- Improve systicks count algorithm during delays.
SystemInit is removed from TI startup files.
Clock initialization is called during initialization with  __libc_init_array.
Update Readme.md : add LTO option documentation
use the same linker script for stm32 boards
use the same linker script for all boards
user can select the optimisation level (in release mode) by adding -DOPTIM_LEVEL=x on the cmake command line
uart_callback is now a std::function
add a helper to include drivers files
begin of zigbee support
Rename TI HAL files.
Refactor TI GPIO HAL.
Modify TI startup and syscall to match common linker script.
Fix arduino GPIO example to new GPIO HAL.
Add functions in TI and ST GPIO HAL.
Add UART support for TI :
 - Simple RX/TX seems to work;
 - Hardware control flow is not tested yet;
 - Callback is not implemented yet.
Change UART number in uart_example to be compatible with TI lm4f120xl.