add pit-core driver and use pit framework in sam7s pit
kernel: scheduler: add start_schedule
kernel: main: call start_schedule
mach-at91: pit: fix bug in enable pit
arm: kernel: add svc.S file
boot: sam7s: define system/user stack
kernel: main: make task never finish
use switch_context, init sp reg in create task
scheduler: implement activate_context on first task scheduling
arm: kernel: svc: implement activate_context code
boot: sam7s: fix init sys mode stack
include: task: task_count don't have to static
arm: kernel: implement svc_create_context