Move loader.mk to mk/rules and rmove misleading target.mk file

Fix #65
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.
Merge pull request #75 from georgekang/master

resolve the ipc_deliver blocking issue

Fixes #72

Fix typed message is not copied to receiver. Ipc can be issued only when
both sender and receiver are in blocked state.
trivial indentation fix