Let compile and link separately
But cannot find reset handler
warning: cannot find entry symbol Reset_Handler; defaulting to 0000000008000000
Fix 'no reset handler issue', not test yet
This commit have fixed the most of part of linking error.
1)`---nostartfiles` option conflict with `CFLAGS_NEW_LIB_NANO`
2)only link mathlib, no standard lib(we already have used newlib-nano)
3)the no reset hanlder issue is dangerous. Actually, I found we only link one object file to elf. I didn't set the variable of objs correctly. This warning implied linker could not find any implementation part of reset handler.
Merge branch 'os_dev' into makefile_improve

Conflicts:
	program/Makefile
Delete useless splash
Support openocd to flash program
Merge pull request #142 from moon-bear/makefile_improve

Makefile improve