How can I manually add the line CONFIG_XILINX_FIXED_DEVTREE_ADDR=y to the linux config file? It keeps getting overwritten when I build the kernel
You can build by make CONFIG_XILINX_FIXED_DEVTREE_ADDR=y without adding it into .config. But if you gonna add it to .config, you should use make menuconfig to select it and save it into .config.