libbootloader_message: convert to Android.bp

Test: build
Change-Id: Ia0c2e141673e37eea29306817d2f4b2c944213b0
diff --git a/Android.bp b/Android.bp
index f919ebc..49438ad 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,3 +1,4 @@
 subdirs = [
+    "bootloader_message",
     "otautil",
 ]