mini-os: Reorganised arch headers

Before, we had:

- include
- include/arm
- include/x86

Where either arm or x86 was added directly to the search path. So
<x86/arch_limits.h> was the same file as <arch_limits.h> on x86.

We now have:

- include
- arch/arm/include
- arch/x86/include

This means that only the second form is now legal. It also simplifies
installation of the headers, since we can now install everything in the
include directory without getting files for other archs.
mini-os: Removed unused (and missing) header
mini-os: Expose opt_console_dev and xenstore_buf for Mirage