[pvmfw][microdroid] Make empty_file filegroup for reuse

Test: m pvmfw_test microdroid_initrd_debuggable \
 microdroid_initrd_normal
Change-Id: I9767ee97900e1b9fa60c45eee8556012cb9e0c20
diff --git a/pvmfw/Android.bp b/pvmfw/Android.bp
index 0d845f9..7f7787d 100644
--- a/pvmfw/Android.bp
+++ b/pvmfw/Android.bp
@@ -32,6 +32,13 @@
     ],
 }
 
+// Generates an empty file.
+genrule {
+    name: "empty_file",
+    out: ["empty_file"],
+    cmd: "touch $(out)",
+}
+
 cc_binary {
     name: "pvmfw",
     defaults: ["vmbase_elf_defaults"],
@@ -81,7 +88,7 @@
             src: ":pvmfw_bin",
         },
     },
-    src: "empty_file",
+    src: ":empty_file",
     installable: false,
 }