ART: Allow to change boot image pickup order

Allow to change the pickup order of boot image files between
system-first and data-first.

Bug: 126307038
Test: m test-art-host
Test: Boot device with image in /data/dalvik-cache
Change-Id: Id80cfc06aeb023559e1a3706833e57ba4880f43b
diff --git a/runtime/runtime_options.def b/runtime/runtime_options.def
index 9b4aa0f..41f0634 100644
--- a/runtime/runtime_options.def
+++ b/runtime/runtime_options.def
@@ -155,4 +155,8 @@
 RUNTIME_OPTIONS_KEY (Unit,                OnlyUseSystemOatFiles)
 RUNTIME_OPTIONS_KEY (unsigned int,        VerifierLoggingThreshold,       100)
 
+RUNTIME_OPTIONS_KEY (gc::space::ImageSpaceLoadingOrder, \
+                     ImageSpaceLoadingOrder, \
+                     gc::space::ImageSpaceLoadingOrder::kSystemFirst)
+
 #undef RUNTIME_OPTIONS_KEY