Update to r42827:

This patch does the following:

comments author names
incorporates "9.4. Kernel Drivers, Subsystems, and Modules" into "9.2. Why Build a Custom Kernel?"
fixes reference to 9.4 in bsdinstall chapter
puts the configuration file section before the building the kernel section
makes it clearer how to get src and cp the needed config file
general wordsmithing to make the first half of this chapter less confusing

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D4965