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 <geissonator@yahoo.com>
Change-Id: I3c7a9375fb3682ddacb1a86ea25e4f1f571b7dc8
diff --git a/build-setup.sh b/build-setup.sh
index 7a5cacf..1a8c1f9 100755
--- a/build-setup.sh
+++ b/build-setup.sh
@@ -25,7 +25,7 @@
 #                     Default: "", no options set
 #  build_dir          Path where the actual BitBake build occurs inside the
 #                     container, path cannot be located on network storage.
-#                     Default: "/tmp/openbmc"
+#                     Default: "$WORKSPACE/build"
 #  distro             The distro used as the base image for the build image:
 #                     fedora|ubuntu
 #                     Default: "ubuntu"
@@ -83,7 +83,7 @@
 UBUNTU_MIRROR=${UBUNTU_MIRROR:-""}
 
 # Docker Image Build Variables:
-build_dir=${build_dir:-/tmp/openbmc}
+build_dir=${build_dir:-${WORKSPACE}/build}
 distro=${distro:-ubuntu}
 img_tag=${img_tag:-latest}
 target=${target:-qemu}