Specifically, disable interrupts just before returning and then enable
the timer interrupt if appropriate. This means that the timer interrupt
cannot actually fire until after the RETI, so the function cannot be
entered recursively.
This costs 2 bytes of ram, but saves 60 bytes of flash. Doing so
also eliminates the need to disable interrupts while clearing flags
in the ifclock macro.
Conflicts:
clock.c
timer.c
timer.h
Jim McGee
Michael Moon
Stephan Walter
Markus Hitter