jenkins/build-seed: remove build directory

The seed script was not removing the build_dir between each
machine.  This causes the conf directory to have stale contents
from older machines and fail to build.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iad1990bd17532d57359c6a01032c7a18a4412ead
diff --git a/jenkins/build-seed b/jenkins/build-seed
index 9bc8fd0..7bf1ce0 100755
--- a/jenkins/build-seed
+++ b/jenkins/build-seed
@@ -40,4 +40,5 @@
     "${WORKSPACE}/openbmc-build-scripts/build-setup.sh" || \
         echo "Build failed for $m; continuing."
     rm -rf "${WORKSPACE}/openbmc/build"
+    rm -rf "${build_dir}"
 done