pvmfw.img: Generate an AVB-signed image

Use a prebuilt_etc to be able to import it in make through

    $(call module-target-built-files,pvmfw_sign_key)

Bug: 237371962
Test: m pvmfw_img
Change-Id: Ie42f3e7b39e461b0baad5bd1ad51ef985c871976
diff --git a/pvmfw/Android.bp b/pvmfw/Android.bp
index 57a3df0..84cb18c 100644
--- a/pvmfw/Android.bp
+++ b/pvmfw/Android.bp
@@ -43,6 +43,12 @@
     },
 }
 
+prebuilt_etc {
+    name: "pvmfw_sign_key",
+    src: ":avb_testkey_rsa4096",
+    installable: false,
+}
+
 bootimg {
     name: "pvmfw_img",
     stem: "pvmfw.img",
@@ -55,4 +61,6 @@
             enabled: true,
         },
     },
+    use_avb: true,
+    avb_private_key: ":pvmfw_sign_key",
 }