make keil/gcc it's own scons tool

This makes keil/gcc it's own scons tool in RTT. It will minimize the job
of rtconfig.py and building.py. By making the configuration of tool
chains in one place, the tool chain auto-detection is also possible for
all bsps.

This commit only updated the stm32f10x and stm32f40x bsp and only
support keil/gcc so far.