Merge pull request #71 from georgekang/master

mk: move loader.mk to directory rules and remove misleading target.mk file
Use enum value instead hardcode in system call funtcions
Add space to improve readability in system call inline assembly
Merge pull request #73 from zzz0072/master

user: annotate enum value instead of hardcode for syscalls
Write typed ipc message to destination thread
Check sender state in ipc_deliver

If sender is not at blocked state, we can not do ipc between
sender and receiver.
Handle blocked sender in ipc_deliver

Fix#72 to avoid ipc hanging.