update the bram contents of bitstream for changes in the bootrom code

Created by: furkanturan

This is a commit to update the bitstream contents for the changes on bootrom code, without resynthesizing the design. It takes seconds to update the bitstream, so speeds up the baremetal application development.

The steps it follows are given below:

• run.tcl reports all the bram instances and their locations to reports/ariane.bram.log
• make $(bit) (in fpga dir) finds the target brams for bootrom, and reports them to work-fpga/ariane_xilinx.bootrom.mmi
• make fpga_update (in top dir) executes the make bram_update (in fpga dir). That starts with executing the same make target in fpga/src/bootrom to compile the boot code and generate corresponding bootrom.mem file. Then, it executes the updatemem tool to replace the bram contents of the bitstream with this mem file.

The updated bitstream is written to work-fpga/ariane_xilinx_updated.bit.

For making this possible, the structure of bootrom.sv is changed. The 64-bit words of the bootrom code are divided into two 32-bit words, which are concatenated later. This implementation forces Vivado to instantiate two BRAMs each are 32-bit in size. These two BRAM contents are updated with mem file. If the bootrom content is not divided to two 32-bit words by hand, then Vivado does the division itself. In that case, it divides the contents to two BRAMs respectively having sizes of 36-bits and 28-bits. In this case, the first BRAM stores 32-bit of the words into its data storage, while the remaining 4-bits in into parity storage. Furthermore, the updatemem tool is not capable of updating the parity storage. As a result, we prefer to make the bootrom implementation by hand to avoid un-updateable BRAM use.