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.

Dict Comprehensions is imported in Python 2.7 & 3.0 which is PEP 274 [1]. Some
OS like CentOS 6 which is use Python 2.6 cannot support this syntex.

[1] http://legacy.python.org/dev/peps/pep-0274/Signed-off-by: NXiongfei Guo <xfguo@credosemi.com>

The startupinfo does not have any visible effects on the building
process and some old Python version does not have it at all. So remove
it for the seek of simple.

Tested `scons -j4` in a dual core virtual machine.