Merge pull request #18 from gogojesse/scheduler_dev

Set system timer to 10 ms.
Add mutex

Signed-off-by: joshych <yucheng1102@gmail.com>
Merge pull request #13 from gogojesse/mutex_dev

mutex implementation
fixes #19
Save/Restore both lr and pc while context switch

Signed-off-by: gogojesse <gogojesseco@gmail.com>
Add sample code for mutex mechanism.

Signed-off-by: gogojesse <gogojesseco@gmail.com>