manta: set default USB configuration to "MTP"

Bug: 6679671
Change-Id: Iec3c64f7442c1e2009a4a2110891a025086a99e2
diff --git a/device.mk b/device.mk
index a526cd8..8d41a16 100644
--- a/device.mk
+++ b/device.mk
@@ -135,6 +135,10 @@
     dalvik.vm.heapgrowthlimit=64m \
     dalvik.vm.heapsize=256m
 
+# set default USB configuration
+PRODUCT_DEFAULT_PROPERTY_OVERRIDES += \
+    persist.sys.usb.config=mtp
+
 # for off charging mode
 PRODUCT_PACKAGES += \
     charger \