KERNEL_SOURCE.txt: Add README for kernel building

Provide some hints and guidance on how customers can build kernels
that they can boot on NI targets.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
nibb.sh: Fix issue with MACHINE assignment

The way in which the machine was being assigned in auto.conf would
clobber other assignments, including the one in the environment from
sourcing the environment file.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
README/KERNEL_SOURCE.txt: Apply feedback from TechPub

Signed-off-by: Brad Mouring <brad.mouring@ni.com>