[U-Boot] Riscv: CONFIG_DEFAULT_DEVICE_TREE failures

I attempted to merge in the latest master branch into https://github.com/sifive/u-boot/tree/sandbox and I got the following error, which seems somewhat broken..

Device Tree Source is not correctly specified. Please define 'CONFIG_DEFAULT_DEVICE_TREE' or build with 'DEVICE_TREE=<device_tree>' argument

dts/Makefile:28: recipe for target 'arch/riscv/dts/fu540.dtb' failed

Does anyone have any idea what would have caused that?

Also any feedback or pointers on why the serial_sifive.c driver with CONFIG_DM_SERIAL would not work, but serial_hifive.c without CONFIG_DM_SERIAL does seem to work might be appreciated.