support installing any .zip file on the sdcard

Replaces the "install sdcard:update zip" menu option with one that
displays a menu of zip files (and subdirs) on the sdcard and lets you
pick which one to install.

Change-Id: I85c94c0e9bc8e05ca52031fc29ca2624c2695ced
diff --git a/default_recovery_ui.c b/default_recovery_ui.c
index 409d679..ce12787 100644
--- a/default_recovery_ui.c
+++ b/default_recovery_ui.c
@@ -24,7 +24,7 @@
                          NULL };
 
 char* MENU_ITEMS[] = { "reboot system now",
-                       "apply sdcard:update.zip",
+                       "apply update from sdcard",
                        "wipe data/factory reset",
                        "wipe cache partition",
                        NULL };