1. Add targets into build_travis.py and tests.py.
2. Add target SPI pins into SPI SD test samples.
3. Rename target TOOLCHAIN_GCC_ARM/retarget.c to avoid name collision of compiled retarget.o with platform/retargets.cpp.
1. Move M451Series.h to device.
2. Move StdDriver to device.
3. Move TOOLCHAIN_* to device.
4. Move cmsis*, startup_M451Series.c, and system_M451Series.c/h to device.