refactor(aarch64,riscv64): unify kernel image detection

[Gelbpunkt]

This unifies the kernel image detection for both aarch64 and riscv64. riscv64 was incorrectly using the length of the module reg as the size of the kernel image, which was always 0, which is fixed by using the approach taken by the aarch64 code to parse the ELF header. On the other hand, aarch64 now falls back to linux,initrd-start and linux,initrd-end for detecting the kernel image thanks to these changes, which is necessary for cloud-hypervisor at the moment.

[mkroening]

We already have similar code for RISC-V: https://github.com/hermit-os/loader/blob/v0.5.6/src/arch/riscv64/mod.rs#L21-L51

It might make sense to unify them as methods on an trait FdtExt, but that might not be worth the effort. You can decide. :)

[Gelbpunkt]

Ah, I didn't even bother checking the RISC-V code. That one is also slightly prettier than this approach. I'll try the FdtExt approach, that seems sensible.

[Gelbpunkt]

CI on aarch64 fails because the fdt is... invalid as far as I can tell. size-cells is set to 2 and address-cells set to 2, but the inserted module only has 32-bit components, i.e. is for size-cells 1 and address-cells 1. Not sure yet what's responsible for this, but it was already an issue before and the code just didn't properly parse/validate it