[OXT-23] Move local build config to openxt.git and use build/ as TOPDIR ...

...for the OXT OE build.

This commit moves the local build metadata out of the xenclient-oe.git repo
and into the openxxt.git repo. These things aren't typically part of a meta
layer so if we're turning xenclient-oe.git into a meta layer then these need
a new home in openxt.git. Further minor edits to these files have been made
so that the build TOPDIR is build/ instead of build/oe/.

bb: Move the helper script 'bb' from xenclient-oe.git to the the openxt.git
    build directory.
bblayers.conf: Move the main 'conf' directory and the bblayers.conf file from
    xenclient-oe.git to openxt.git.
do_build.sh: Fixup paths to find the new location of the build output and
    configuration.
.gitignore: Be a bit more specific about ignored build
    files.