boot from external flash on genesys2 fpga

[a0000442]

hi
Can anyone tell me how to boot from external flash on geneysy2 fpga board in detail？
I use pulpissimo 7.0.0 and genesys2 fpga board.I can download app through openocd but
I cannot boot it from external flash.
I konw the step how to boot from flash
1. set the bootsel=2b00；
2. modify the bootrom.c in boot_code 
3. make and convert bootrom.c to fpga_bootrom.sv
4. rerun the fpga process and burn it
5. use plp_mkflash to convert app to flash.bin
6. burn flash.bin to external flash 
7.reset fpga and run app in flash
is it correct？
can anybody help me？how to modify bootrom.c in step2？