boot: refactor allocation of rootserver objects

Prior to this change, the boot process would dynamically allocate
memory for root server objects based on the order of initialisation.
Allocation was a best-fit algorithm.

This change preallocates all memory for root server objects to an
aligned untyped just after the user image. By allocating the objects in
order of size, allocation is greatly simplified and the ability to
reproduce the allocation offline based on the kernel and user image
sizes is increased.