Add cconfig.h file for different version of GCC, which is automatically
detected and generated by scons script.

Example "rtconfig.h" content:
 ...
 //#define BOARD_USING_LED
 #if defined(BOARD_USING_LED)
 #define RT_USING_RTGUI
 #endif
 ...

- Before patching, RTGUI module will be built although not intend to do so.
- After patching, RTGUI module will not be built.

The global variables in the building.py are totally a pile of shit.

Add bsp_directory for PrepareModuleBuilding function and module building environment can parse rtconfig.h too.

The source files can be the raw file names end with .c or .o. So return
the raw file names is more flexible than wrap them with Object.

If the src is empty list, it will crash at:

    IndexError: list index out of range:
      File "/home/xxx/src/SConstruct", line 39:
        objs = PrepareBuilding(env, RTT_ROOT, has_libcpu=True)
      ...
      File "/home/xxx/src/drivers/SConscript", line 12:
        SrcRemove(src, src_need_remove)
      File "/home/rt-thread-stable/tools/building.py", line 496:
        if type(src[0]) == type('str'):

The order of them are important while the uniqueness has nothing to do
with the compiling.