soong: Add genrule_defaults for 7z prebuilts

Change-Id: I9efc0b32aeb02faa63635a1f5e26f74debcbf4fd
diff --git a/build/soong/Android.bp b/build/soong/Android.bp
index cff25e4..ddedb0a 100644
--- a/build/soong/Android.bp
+++ b/build/soong/Android.bp
@@ -56,6 +56,12 @@
     ],
 }
 
+genrule_defaults {
+    name: "7z_genrule_defaults",
+    tools: ["7z"],
+    cmd: "$(location 7z) e -so $$(echo $(in) | cut -f1 -d ' ') > $(out)",
+}
+
 cc_library_headers {
     name: "generated_kernel_headers",
     generated_headers: ["generated_kernel_includes"],