Export soong docs

Copy soong_build.html to DIST_DIR so that we have somewhere to get an
up-to-date copy of the docs.

Change-Id: I610cc75c2508ee48b7b372d268c75cd6c4bf4f63
diff --git a/build-prebuilts.sh b/build-prebuilts.sh
index ee2608a..0ece7dd 100755
--- a/build-prebuilts.sh
+++ b/build-prebuilts.sh
@@ -77,6 +77,7 @@
 
     if [ -n ${build_soong} ]; then
         cp ${SOONG_HOST_OUT}/build-prebuilts.zip ${DIST_DIR}/
+	cp ${SOONG_OUT}/.bootstrap/docs/soong_build.html ${DIST_DIR}/
     fi
     if [ -n ${build_go} ]; then
         cp ${GO_OUT}/go.zip ${DIST_DIR}/