add function for device-specific wipe data features

Some devices want to do special things when recovery wipes data (eg,
wipe data in their baseband processor as well).  Add a hook in the
device-specific recovery library that gets called when data is wiped.

Also add an amend root for the "mbm" partition.
diff --git a/default_recovery_ui.c b/default_recovery_ui.c
index 76fa43e..a2e4bea 100644
--- a/default_recovery_ui.c
+++ b/default_recovery_ui.c
@@ -59,3 +59,7 @@
 int device_perform_action(int which) {
     return which;
 }
+
+int device_wipe_data() {
+    return 0;
+}