busy_wait_at_least_cycles() function

Helper method to busy-wait for at least the given number of cycles This method is useful for introducing very short delays. This method busy-waits in a tight loop for the given number of system clock cycles. The total wait time is only accurate to within 2 cycles, and this method uses a loop counter rather than a hardware timer, so the method will always take longer than expected if an interrupt is handled on the calling core during the busy-wait; you can of course disable interrupts to prevent this. You can use clock_get_hz(clk_sys) to determine the number of clock cycles per second if you want to convert an actual time duration to a number of cycles.