avr: Implement internal avr specific timer to handle 16bit overflows

Don't rely on the generic scheduler code to always have a timer no
more than 1ms in the future.  Instead, create an avr specific timer
that will be called every 0x8000 ticks.  This simplifies the generic
code and it reduces the amount of code that needs to be run every
millisecond.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/misc.h

@@ -12,7 +12,6 @@ uint32_t timer_from_us(uint32_t us);
 uint8_t timer_is_before(uint32_t time1, uint32_t time2);
 uint32_t timer_read_time(void);
 void timer_kick(void);
-void timer_periodic(void);
 
 void *dynmem_start(void);
 void *dynmem_end(void);