Fix bootstrap.bash -r with Android.bp

Change-Id: I8182e57105b1c17254e082f6b72d2504befb7870
diff --git a/bootstrap.bash b/bootstrap.bash
index 551ba72..e9a6436 100755
--- a/bootstrap.bash
+++ b/bootstrap.bash
@@ -2,6 +2,7 @@
 
 export BOOTSTRAP="${BASH_SOURCE[0]}"
 export SRCDIR=$(dirname "${BASH_SOURCE[0]}")
+export TOPNAME="Android.bp"
 export BOOTSTRAP_MANIFEST="${SRCDIR}/build/soong/build.ninja.in"
 
 case $(uname) in