Allow bionic to specify its own root Blueprints

subdirs = ["bionic/*"] in the root Blueprints causes blueprint to
search subdirectories of bionic for Blueprints files instead of
searching the bionic directory itself.  Replace it with
subdirs = ["bionic"] and allow bionic/Blueprints to specify its
subdirs.

Change-Id: Iac9d2407a3d4c87b3c23390f887b187f276ee72f
diff --git a/root.bp b/root.bp
index 760e5dc..00ee1f0 100644
--- a/root.bp
+++ b/root.bp
@@ -2,7 +2,7 @@
     "art",
     "build/blueprint",
     "build/soong",
-    "bionic/*",
+    "bionic",
     "external/*",
     "libnativehelper",
     "prebuilts/ndk",