Add offset option to freedom-bin2hex script for supporting SPI flash

[kenzhang82]

In freedom e310 arty kit, the bootROM doesn't jump to the base address of the SPI flash address, but with an offset, add the offset option such that 0's are prepend.

[kenzhang82]

@palmer-dabbelt can you please review this?

[bsousi5]

An issue with this approach, is you will overwrite the existing contents prior to you program offset.
Consider the following;

[---------------- FLASH MEMORY ------------------------]
| [ FPGA bits ] -- [ BootROM ] --- [ User Program ] ---|
[------------------------------------------------------]

Prepend with leading 0s will overwrite the FPGA bits region

[nategraff-sifive]

@palmer-dabbelt can you please review this?

Palmer doesn't work at SiFive anymore (😢) so you'd be hard pressed to get his review on this. (Palmer, if you do see the notification, hello! Hope you're well!) Bunnaroath and I are the maintainers of Freedom E SDK.

[kenzhang82]

Hi @nategraff-sifive, thanks for letting me know that Palmer doesn't work at SiFive anymore.

Hi @bsousi5, thanks for the review and feedback, I wonder how we should go handle the case of RTL simulation where the FPGA bitstream and bootROM doesn't really matter. Maybe add a simulation mode option to the script?