Add microdroid, a minimal Android-based Linux image

microdroid is a minimal Linux image that we will use in VMs. Instead of
inventing a whole new Linux image from scratch, we design microdroid as
a stripped down version of the generic Android system image (GSI).

Bug: 177630284
Test: m microdroid
Test: no runtime test. This isn't mountable yet.
Change-Id: I528e9da2a0933e7d024f3606c82df585d029785c
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
new file mode 100644
index 0000000..fd2c6c2
--- /dev/null
+++ b/microdroid/Android.bp
@@ -0,0 +1,9 @@
+android_filesystem {
+    name: "microdroid",
+    use_avb: true,
+    avb_private_key: "microdroid.pem",
+    avb_algorithm: "SHA256_RSA4096",
+    deps: [
+        "init_second_stage",
+    ],
+}