doc: Purge inappropriate fundamentals info from Kernel Primer

Incorporates the brief discussion of device driver support into
the Device Drivers document, so that the driver material is all
in one place. (Also converts a few unprintable characters in the
latter document to spaces, since they appear to have been used
by accident.)

Removes the empty discussion of networking support, to avoid
duplicating stuff already covered in the Networking document.

Change-Id: Ia5b8a92ade72a0634ee142afb45928016442d7dd
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>