In section 3.1, the docs imply that I can make a site.conf file to configure multiple builds simultaneously.  It also indicates that I can "extend your BBPATH variable to include the path to a conf/site.conf. Then, when BitBake looks for Metadata using BBPATH, it finds the conf/site.conf file and applies your common configurations found in the file."

But, the default bblayers.conf does a BBPATH = "${TOPDIR}" which means that any BBPATH env settings I have are wiped out. Having to append the bbpath in each bblayers.conf file seems to somewhat obviate the advantages of a site.conf file. 

For my curent usage model, changing the default bblayers.conf to do BBPATH .= ":${TOPDIR}" works, but it breaks sanity check if you don't have BBPATH in your env since you end up with an empty :: which is the same as having a "." in BBPATH.

Initial ideas on how to address:
1) use BBPATH .= ":${TOPDIR}" in bblayers.conf and then strip out any :: in bbpath.
2) add an additional var BBPATH_SITE to list sitewide places to search