kernel: Enhance memory pool block allocation routine

Revises algorithm for allocating an existing free block to make it
more compact (and efficient) by using the same logic to handle the
allocation of any block within a quad-block.

Also enhances routine name and comments to make it easier to
understand what is going on.

Change-Id: I55ca513e6f85df2b548502262e4dbe6bb272596b
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>