Working F401 and DISCO F407 device.mk files

I sent longer emails, but I believe these STM device.mk files are OK now.
The floating point options on ASM_FLAGS is required because RTOS delivers an assembly file with floating point instructions.

Setting -mfloat-abi=softfp lets compiler link your main.c to either software FP or hardware FP  compiled libraries in one run. The setting hard requires the entire lib to be compiled with -mfloat-abi=hard including the gcc's libs.

The -fsingle-precision-constant is to advise the compiler that the FPU is only 32bit. It should generate soft for double precision. The user must use single precision calls such as sinf, tanf to get hardware fp code.