generalize "install from sdcard" to "install from external storage"

Allow sideloading of OTA packages from USB drives that appear as
/dev/block/sda1.

Change-Id: I1908576c24547cd0088475d8c8917699cd906868
diff --git a/default_recovery_ui.c b/default_recovery_ui.c
index ce12787..bcba888 100644
--- a/default_recovery_ui.c
+++ b/default_recovery_ui.c
@@ -24,7 +24,7 @@
                          NULL };
 
 char* MENU_ITEMS[] = { "reboot system now",
-                       "apply update from sdcard",
+                       "apply update from external storage",
                        "wipe data/factory reset",
                        "wipe cache partition",
                        NULL };