dev-manual, ref-manual: Updates for BBPATH.

dev-manual, ref-manual: Updates for BBPATH.

Two things fixed:

1. A couple examples for setting up a layer.conf file were
   using the meta-yocto configuration file as an example.
   Turns out this is a bad example because the BBPATH variable
   is appended to the LAYERDIR when setting BBPATH rather
   than the other way around.  That is because meta-yocto
   is a distro layer.  BSP layers, however, are not distro
   layers and are the norm here.  So I switched up the way
   BBPATH was set and actually called out meta-yocto-bsp
   config file as a template.

2. Added a link for the BBPATH variable in the
   FILESYSTEM_PERMS_TABLES variable description in the
   glossary.  Just happened to notice this so I fixed it

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
(From yocto-docs rev: 1a594b6dde1a494a5f7322b510e1e67d23635298)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>