simulator: move SDL setting staff into drivers/SConstruct

This could not only make a cleaner SConstruct, but could also benefit
RTGUI building.
simulator: get CROSS_TOOL and EXEC_PATH from env

Simulator get it's way to Travis CI ready.
Scons: run `clang -Wall -fsyntax-only` in clang-analyze

The `clang -fsyntax-only` will give us additional warning messages in
the console.
Scons: add a custom tool to do clang static analyze

Bsps can use the clang analyzer as a tool:

    env = Environment(toolpath=[os.path.join(RTT_ROOT, 'tools',
                      'tools')], tools = ['clang-analyze'])

When building the project, the static analyzer will be called to check
all the C code. The warnings are print to stderr.
simulator: add clang-analyze tool support

When CROSS_TOOL or 'RTT_CC' env is setted to 'clang-analyze', it will
use Clang to check the codes.
travis: auto build the simulator

Building is only the first step. Correctness is what we need. There are
already many GCC builds for other bsp so GCC building for simulator is
not important. So I use clang-analyze to check all the source codes in
simulator project. Hope it will help us.
travis: try to fix the SDL building
travis: install the gcc-multilib to get the 32bit build OK
travis: don't export RTT_CC in before_script if it's been set

This gives ways for bsp to set it's own RTT_CC.
travis: add scan-build in PATH
Merge pull request #155 from grissiom/simulator

Simulator