kernel image fields & zero out rbp

[devsnek]

these changes stemmed from my efforts to add stack traces to my kernel

• kernel_addr + kernel_len for building a slice that holds the elf
• kernel_image_offset for computing offsets from the instruction pointers in stack frames
• convention says that you should zero rbp before jumping into your actual code so that you know where to stop the stack trace

[phil-opp]

Thanks a lot for this PR! Looks good overall, but we need to be more explicit about the different address spaces.

[phil-opp]

There is some prior discussion about a similar change in #177. Back then I argued that might not want to guarantee that we keep the full kernel ELF file in physical memory and instead make it configurable. My original thought was that we might want to optimize the memory consumption. However, we haven't implemented such an optimization in the past two years, so I think I'm fine with merging this as-is now.

[devsnek]

@phil-opp is anything still remaining here? could it be merged?

[phil-opp]

Thanks for the adjustments! Looks good, sorry for the slow review.