Replace soong script with a symlink

bootstrap.bash creates a soong script in the output directory using
build/soong/soong.in.  This requires a manual rebootstrap any time
soong.in changes.  Instead, have bootstrap.bash symlink
build/soong/soong.bash to soong in the output directory, and create
a file called .soong.bootstrap in the output directory that contains
the variables that bootstrap.bash sets.

Change-Id: I5e6e54c2e8bdde876941e2e082f9ba177c757cbf

bootstrap.bash

@@ -27,8 +27,8 @@ fi
 if [[ $# -eq 0 ]]; then
     sed -e "s|@@SrcDir@@|${SRCDIR}|" \
         -e "s|@@PrebuiltOS@@|${PREBUILTOS}|" \
-        ${SRCDIR}/build/soong/soong.in > soong
-    chmod 0770 soong
+        ${SRCDIR}/build/soong/soong.bootstrap.in > .soong.bootstrap
+    ln -sf ${SRCDIR}/build/soong/soong.bash soong
 fi
 
 ${SRCDIR}/build/blueprint/bootstrap.bash "$@"