cm: bootanimation: Fail the build if mogrify is absent

We're no longer using convert, but mogrify.

Change-Id: Ifd58527b49e5548dd94b35a8cf0ffa4ad5196804

bootanimation/Android.mk

 #
 # Copyright (C) 2016 The CyanogenMod Project
+#               2017 The LineageOS Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -28,7 +29,7 @@ $(TARGET_GENERATED_BOOTANIMATION):
 
 ifeq ($(TARGET_BOOTANIMATION),)
     TARGET_BOOTANIMATION := $(TARGET_GENERATED_BOOTANIMATION)
-    ifeq ($(shell command -v convert),)
+    ifeq ($(shell command -v mogrify),)
         $(info **********************************************)
         $(info The boot animation could not be generated as)
         $(info ImageMagick is not installed in your system.)