Added a very empty but compiling source

src/main.c

src/startup_stm32f40xx.s

src/stm32f4xx.h

src/stm32f4xx_conf.h

src/stm32f4xx_it.c

src/stm32f4xx_it.h

src/syscalls.c

src/system_stm32f4xx.c

src/tiny_printf.c