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

soong.bash

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+# Determine the build directory location based on the location of this script.
+BPBUILD="${BASH_SOURCE[0]}"
+BUILDDIR=`dirname "${BASH_SOURCE[0]}"`
+BOOTSTRAP=${BUILDDIR}/.soong.bootstrap
+
+# The source directory path and operating system will get written to
+# .soong.bootstrap by the bootstrap script.
+
+if [ ! -f ${BUILDDIR}/${BOOTSTRAP} ]; then
+    echo "Error: soong script must be located in a directory created by bootstrap.bash"
+    exit 1
+fi
+
+source ${BUILDDIR}/.soong.bootstrap
+
+if [[ ${SRCDIR_IN:0:1} == '/' ]]; then
+    # SRCDIR_IN is an absolute path
+    SRCDIR=${SRCDIR_IN}
+else
+    # SRCDIR_IN is a relative path
+    SRCDIR=${BUILDDIR}/${SRCDIR_IN}
+fi
+
+# Let Blueprint know that the Ninja we're using performs multiple passes that
+# can regenerate the build manifest.
+export BLUEPRINT_NINJA_HAS_MULTIPASS=1
+
+${SRCDIR}/prebuilts/ninja/${PREBUILTOS}/ninja -C ${BUILDDIR} "$@"