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/ui.c b/ui.c
index 01a005f..ef21257 100644
--- a/ui.c
+++ b/ui.c
@@ -425,7 +425,7 @@
     pthread_mutex_unlock(&gUpdateMutex);
 }
 
-void ui_start_menu(char** headers, char** items) {
+void ui_start_menu(char** headers, char** items, int initial_selection) {
     int i;
     pthread_mutex_lock(&gUpdateMutex);
     if (text_rows > 0 && text_cols > 0) {
@@ -442,7 +442,7 @@
         }
         menu_items = i - menu_top;
         show_menu = 1;
-        menu_sel = 0;
+        menu_sel = initial_selection;
         update_screen_locked();
     }
     pthread_mutex_unlock(&gUpdateMutex);