Preparation for long descriptor page table support
This is a selection of commits from !15 (closed), getting the kernel ready for the addition of long descriptor page table support.
Mostly this focuses on moving some code around (most low-level page table manipulation is now in the new file s/ShortDesc), and refactoring other code to reduce the number of places where page tables are accessed directly (use subroutines instead, like logical_to_physical). It's probably best to review each commit in turn to avoid confusion.
From OS/software point of view the kernel should look and function exactly the same as before.
Each individual commit builds for IOMD, Tungsten, and a random ARMv6+ target.