Disable microdroid_kernel_signed for non-arm64/x86_64 architectures

avb_add_hash_footer fails when the image/src is empty. Virt apex is not
compatible with other architectures anyway.

Test: lunch full-eng -> m com.android.virt
Bug: 240235424
Change-Id: Ic476d5bb7f0032aa9404778b50a6cd202a157319
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index 67f3117..294ddf5 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -571,31 +571,36 @@
     srcs: ["bootconfig.normal"],
 }
 
-prebuilt_etc {
-    name: "microdroid_kernel_unsigned",
-    src: "empty_kernel",
-    filename: "microdroid_kernel_unsigned",
-    arch: {
-        arm64: {
-            src: ":microdroid_kernel_prebuilts-5.15-arm64",
-        },
-        x86_64: {
-            src: ":microdroid_kernel_prebuilts-5.15-x86_64",
-        },
-    },
-}
-
 avb_add_hash_footer {
     name: "microdroid_kernel_signed",
-    src: ":microdroid_kernel_unsigned",
+    src: "empty_kernel",
     filename: "microdroid_kernel",
     partition_name: "bootloader",
     private_key: ":microdroid_sign_key",
     salt: bootloader_salt,
+    enabled: false,
+    arch: {
+        arm64: {
+            src: ":microdroid_kernel_prebuilts-5.15-arm64",
+            enabled: true,
+        },
+        x86_64: {
+            src: ":microdroid_kernel_prebuilts-5.15-x86_64",
+            enabled: true,
+        },
+    },
 }
 
 prebuilt_etc {
     name: "microdroid_kernel",
-    src: ":microdroid_kernel_signed",
+    src: "empty_kernel",
     relative_install_path: "fs",
+    arch: {
+        arm64: {
+            src: ":microdroid_kernel_signed",
+        },
+        x86_64: {
+            src: ":microdroid_kernel_signed",
+        },
+    },
 }