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/recovery_ui.h b/recovery_ui.h
index e451bdf..77ce7f9 100644
--- a/recovery_ui.h
+++ b/recovery_ui.h
@@ -66,7 +66,8 @@
 #define SELECT_ITEM         -4
 
 #define ITEM_REBOOT          0
-#define ITEM_APPLY_SDCARD    1
+#define ITEM_APPLY_EXT       1
+#define ITEM_APPLY_SDCARD    1  // historical synonym for ITEM_APPLY_EXT
 #define ITEM_WIPE_DATA       2
 #define ITEM_WIPE_CACHE      3