build-setup: default build dir to under workspace
build-setup: default build dir to under workspace

Newer docker containers are moving to the overlay and overlay2
filesystems by default. bitbake does not work on these types of
filesystems. By default, the $WORKSPACE will always be mounted into the
container and this will have an appropriate filesystem to build upon so
make the default build location a sub-directory under this.

Tested:
Verified an image can be built with newer docker version

Signed-off-by: Andrew Geissler <f7919ac9f8ec0c3ffbc3f483da625c634fd31638@yahoo.com>
Change-Id: I3c7a9375fb3682ddacb1a86ea25e4f1f571b7dc8

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts