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/screen_ui.h b/screen_ui.h
index d8ac85b..4319b76 100644
--- a/screen_ui.h
+++ b/screen_ui.h
@@ -34,6 +34,7 @@
 
     // overall recovery state ("background image")
     void SetBackground(Icon icon);
+    void SetSystemUpdateText(bool security_update);
 
     // progress indicator
     void SetProgressType(ProgressType type);