occamy: Boot from SPI flash

Created by: niwis

This adds/changes the following make targets required to boot Linux on occamy from the SPI flash:

• uImage.bin: U-Boot compatible Linux image. Can be loaded into the VCU128's flash via Vivado.
• fw_payload: Change the OpenSBI payload from Linux to U-Boot.

Furthermore, the u-boot submodule is bumped.

These are the current steps required to boot linux:

1. Load uImage.bin via Vivado into the VCU128's SPI flash.
2. Load Occamy bitstream, execute zero-stage boot loader from bootrom.
3. Connect to occamy via OpenOCD/gdb, load fw_payload.elf (OpenSBI + U-Boot), execute.