Extend the description of the DISTRO variable so that it mentions that
this points to a .conf file under conf/distro and mentions what happens
if the value is left blank.
(From yocto-docs rev: 50f8f0394d8d849e0a227d6c9ffcdc3cccb7e307)
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>