Move all sanity checks in one place

All sanity checks performed by the uVisor core should be grouped together and called before anything meaningful is done.

Checks can be further sub-grouped functionality-wise, like:

``` C
sanity_checks_page_allocator(...);
sanity_checks_box_config(...);
...
```

but the single functions should still be called in a central place, in `vmpu_init_pre()`.

Start by locating vmpu_init_pre() and inventory the sanity checks performed by the uVisor core. Group the checks by functionality while keeping their individual functions, then ensure they are called centrally before other meaningful initialization work. Done means all core sanity checks are invoked from vmpu_init_pre().

Written by the indexing model from the issue text.