Function irq_register_timer_handler(int timer, void (*isr)(void))
lets you change the handler at runtime. This lets us set handlers
without necessarily knowing the name at link-time. The old way of
having weakly-linked tmr0_isr functions should still work.
This is an attempt to fix and cleanup all of the rules, and get rid of
some of the recursion. We still need to invoke make several times
when we're building for more than one board, and some things can get a
bit weird when that happens, but it should generally work. A single
BOARD=foo build should be fully correct, now.
I attempted to keep things compatible, so existing Makefiles that
include libmc1322x/Makefile.include should still work correctly.
set... but I'm not sure what to do about it. The big problem is that I
have to be in user mode to service irqs, but I can't enable and
disable F and I in usermode. All I can do is an swi and then have
handler which lets me enable or disable them (like a mini-syscall).
