Improved automated support for FPGAs

Created by: rosethompson

Automatically sets the following based on config and makefile settings

1. Sets the zsbl cpu clock frequency (SYSTEMCLOCK)
2. Sets the max SPI clock frequency
3. Inherits the location and size of main memory from the fpga config.
4. The FPGA clock (mmcm) generator scripts now set the clock speed based on SYSTEMCLOCK

Todo:

1. Still need to fix makefile dependencies so the FPGA script will regenerate the mmcm if the clock changes. Not sure how this should be done.
2. Finally update the device tree. I think we can return to a single device tree file and have this Makefile create the correct dts for this particular build.