Reference to shengwen1997

Add Makefile & stm32_flash.ld
Consistent with the previous code of FLASH part
Fix source code of Makefile.
Add toolchain path in the Makefile
fix error
fix error
Fix misspell, and Makefile can target.
Add startup_stm32f4xx.s to support Makefile

Because startup_stm32f40xx.s have many bugs.
Merge to fboris
Fix the manual-modification mistake

'Merge' have some problem.
Remove useless startup_stm32f40xx
Merge pull request #3 from zxc2694/master

Instead of using the startup_stm32f40xx.s, I use the startup_stm32f4xx.s to support the makefile.