lib: div: remove check for dividend is 0x80000000

As the dividend is unsigned int, so it can be 0x80000000.

Signed-off-by: Bo Shen <voice.shen@gmail.com>
Makefile: use MAKEFLAGS replace MAKE_OPTION

Signed-off-by: Bo Shen <voice.shen@gmail.com>
Merge pull request #6 from voiceshen/voice/makeflags

Makefile: use MAKEFLAGS replace MAKE_OPTION
Merge pull request #7 from voiceshen/voice/fix_div

lib: div: remove check for dividend is 0x80000000