proto-rfc: replace ELF patching (setvar) with passing a 'microkit_bootinfo_t' structure

[midnightveil]

Related: #411, #362, #248

Similar to how seL4_BootInfo_t works, we should pass a bootinfo structure in the $a0 register through to init(), which can contain physical and virtual addresses of various memory regions, populated by the capDL initialiser; instead of doing this at build time with ELF patching. This helps especially with x86, where paddr are often unknown at runtime.

void init(microkit_bootinfo_t *bi) {
  // etc
}

This structure could look something like:

typedef struct _microkit_bootinfo {
   paddr_t paddrs[...];
   vaddr_t vaddrs[...];
   void *preloaded_data_regions[...];
} microkit_bootinfo_t;

and have everything in the system description file have a numeric ID; or alternatively be a bit more wasteful of memory and pass e.g. an array of memory regions which contains the string-name, and id, the vaddr, possibly the paddr, etc. (essentially, serialising the SDF file).

This would also make it a bit easier for non-microkit users to re-use microkit PDs (e.g. Djawula), as this requires no runtime ELF-patching.