kernel: Rename private memory pool types

Revises the names of 2 types to make them better reflect the
data structures they represent, namely:

- block set: a collection of memory pool blocks of a given size
- quad-block: four contiguous memory pool blocks of a given size

Change-Id: I3e424586e97157eea185fba6836e2e89d10d9cd6
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>