Add ability to show "installing security update"

Add a new command "--security" to boot commands. If this command is
observed as part of BCB, choose a different background text picture
for installing stage in recovery UI. As a result, users will see
"installing security update" instead of "installing system update"
when applying a security update package.

Bug: 27837319
Change-Id: I2e2253a124993ecc24804fa1ee0b918ac96837c5
diff --git a/ui.h b/ui.h
index ca72911..82d95a3 100644
--- a/ui.h
+++ b/ui.h
@@ -39,6 +39,7 @@
     // Set the overall recovery state ("background image").
     enum Icon { NONE, INSTALLING_UPDATE, ERASING, NO_COMMAND, ERROR };
     virtual void SetBackground(Icon icon) = 0;
+    virtual void SetSystemUpdateText(bool security_update) = 0;
 
     // --- progress indicator ---
     enum ProgressType { EMPTY, INDETERMINATE, DETERMINATE };