change interrupt num to 53
change stack size to 512 before system starup
add uart drivers
open msh
remain bug:
   uart just interrupt ones

	save sp to old thread
	clear rt_thread_switch_interrupt_flag
always enable interrupt after rt_hw_context_switch
judeg the type of interrupt in trap_entry, then call handler(Machine timer interrupt of Machine external interrupt)

	enable interrupt after return form rt_hw_context_switch

fix open timer intrrupt

	it will lead to interrupt again in interrupt