kernel: Enhance memory pool initialization routine

Makes a couple of minor optimizations to initialization algorithm:

- Now initializes block status array for largest size blocks once,
  rather than twice.
- Doesn't initialize "count" field of block size descriptors to zero,
  since sysgen ensures that this happens automatically.

Also enhances comments for initialization routine to make it easier
to understand what is going on.

Change-Id: I00d907c0f2a86f5b6ea8a63475b40074fe89357c
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>