CPUClock set to 20MHz and zero flash waitstates

This is a small update to my previous changes. 

I had previously set the CPU core clock to 30Mhz. Unfortunately the LPC810 requires one wait state on flash accesses (default setting) for clocks above 20Mhz. Therefore using 20Mhz at zero flash waitstates should yield the fastest and most predictable code execution.

Added wait state initialization to ```System_init()```