Allow specifying address of boot_info structure.

[Darksecond]

When making a higher half kernel it would be useful to move the boot_info into the higher half. It would also have to remap all the child structures, like memory_map

I imagine it would go into the [package.metadata.bootloader] like physical-memory-offset and kernel-stack-address.

[phil-opp]

Implemented in #101.