Clean and document header files.

- Add missing doc comments to header files
- Move allocate_kernel_args to main.cpp
- Split functions out into pointer_manipulation.h

src/boot/elf.h

@@ -1,3 +1,5 @@
+/// \file elf.h
+/// Definitions and related constants for ELF64 structures
 #pragma once
 #include <stdint.h>