conf/machine/include/README: Add readme to explain cpu tunings

conf/machine/include/README: Add readme to explain cpu tunings

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.

(From OE-Core rev: e138f9f7e48e0af94c5c88045c4f0581cc68248d)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>