Provide path to mainDexClasses

Set MAINDEXCLASSES to the path to mainDexClasses.

Change-Id: I82532e8ea8dc7349178347e080fa0c63078ca9f5
diff --git a/core/config.mk b/core/config.mk
index e615159..930e82b 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -535,6 +535,7 @@
 
 # dx is java behind a shell script; no .exe necessary.
 DX := $(HOST_OUT_EXECUTABLES)/dx
+MAINDEXCLASSES := $(HOST_OUT_EXECUTABLES)/mainDexClasses
 ZIPALIGN := $(HOST_OUT_EXECUTABLES)/zipalign$(HOST_EXECUTABLE_SUFFIX)
 ifndef TARGET_BUILD_APPS
 ZIPTIME := $(HOST_OUT_EXECUTABLES)/ziptime$(HOST_EXECUTABLE_SUFFIX)