Default to a u-boot prebuilt when no bootloader or kernel is provided in VirtualMachineConfig

Bug: b/346675532
Change-Id: Ia6379201c8b06eca485156b74101fdef10a6917d
Test: Run with ChromeOS
diff --git a/apex/Android.bp b/apex/Android.bp
index 43819dc..17b1f9e 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -157,6 +157,7 @@
         "microdroid.json",
         "microdroid_kernel",
         "com.android.virt.init.rc",
+        "android_bootloader_crosvm_aarch64",
     ],
     host_required: [
         "vm_shell",