lib: os: introduce memory blocks allocator

This introduces yet another memory blocks allocator where:

() All memory blocks have a single fixed size.

() Multiple blocks can be allocated or freed at the same time.

() A group of blocks allocated together may not be contiguous.
   This is useful for operations such as scatter-gather DMA
   transfers.

() Bookkeeping of allocated blocks is done outside of
   the associated buffer (unlike memory slab). This allows
   the buffer to reside in memory regions where these can be
   powered down to conserve energy.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>