mini-os: mm: reduce buddy allocator list administration data

mini-os: mm: reduce buddy allocator list administration data

Today the administration data for the buddy allocator's lists consists
of 2 arrays: one pointer array and one list element array for easier
handling of the lists' tails.

Those arrays can be combined into one by dropping the pointer array and
using a different list end indicator.

Add enqueue and dequeue helpers for better readability.

Change the level member type to unsigned int.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>