mini-os: Fixed #ifdef test
mini-os: Made off_t type signed

POSIX requires this.
mini-os: Make .o files depend on includes

Before, only the final link step depended on setting up the includes
directory, making the build unreliable.
mini-os: Added install-ocamlfind target

This installs libminios.a and the linker script to the local ocamlfind
repository, where Mirage can find it.
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