Build vendor_boot image for microdroid

It consists of

* common kernel modules for virtual devices
* a dummy dtb image

Bug: 178978059
Test: m microdroid_vendor_boot-5.10

Change-Id: I7df34ef976b557ff750aa566838f744162caf1a3
diff --git a/microdroid/dummy_dtb.img b/microdroid/dummy_dtb.img
new file mode 100644
index 0000000..9d6cec7
--- /dev/null
+++ b/microdroid/dummy_dtb.img
@@ -0,0 +1 @@
+workaround