This cleans up and/or corrects a few values from machine includes
for consistency with future toolchain sanity checks, and also adds
the TUNEVALID and TUNECONFLICTS to documentation.conf.

Signed-off-by: Peter Seebach <peter.seebach@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Add a new README that covers the basic items used with various cpu
tunings.  The goal is to better help people understand the various
settings and where things should or should not be defined.

Corresponding architecture README files will also be generated to
explain the particulars of architectural tunings.

Also remove the default TUNE_PKGARCH setting in bitbake.conf.  This
was done to ensure an error occurs if an invalid tuning is defined.

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>