Simplify bootstrapping

Follow changes from https://github.com/google/blueprint/pull/121

Change-Id: Icc5003f6e8592a667e1ce5e58361c03997078763
diff --git a/soong.bash b/soong.bash
index 6684681..1dbf4e4 100755
--- a/soong.bash
+++ b/soong.bash
@@ -20,9 +20,6 @@
 # $BOOTSTRAP are correct
 cd ${SRCDIR_FROM_BUILDDIR}
 
-# Run the blueprint wrapper
-BUILDDIR="${BUILDDIR}" SKIP_NINJA=true build/blueprint/blueprint.bash
-
 # Ninja can't depend on environment variables, so do a manual comparison
 # of the relevant environment variables from the last build using the
 # soong_env tool and trigger a build manifest regeneration if necessary
@@ -40,4 +37,4 @@
     fi
 fi
 
-"prebuilts/build-tools/${PREBUILTOS}/bin/ninja" -f "${BUILDDIR}/build.ninja" -w dupbuild=err "$@"
+BUILDDIR="${BUILDDIR}" NINJA="prebuilts/build-tools/${PREBUILTOS}/bin/ninja" build/blueprint/blueprint.bash "$@"