RK3399 Android7.1修改系统默认USB功能

在device\rockchip\common\device.mk中修改persist.sys.usb.config的值，如下将USB功能默认设置为MTP（去掉ADB）：
ifeq ($(strip $(BUILD_WITH_CDROM)),true)
PRODUCT_PROPERTY_OVERRIDES +=                 \
    ro.factory.hasUMS=cdrom                   \
    ro.factory.cdrom=$(BUILD_WITH_CDROM_PATH) \
    persist.sys.usb.config=mass_storage,adb

PRODUCT_COPY_FILES += \
    $(LOCAL_PATH)/init.rockchip.hasCDROM.true.rc:root/init.$(TARGET_BOARD_HARDWARE).environment.rc
else
PRODUCT_PROPERTY_OVERRIDES +=       \
    ro.factory.hasUMS=false         \
    persist.sys.usb.config=mtp  \
    testing.mediascanner.skiplist = /mnt/shell/emulated/Android/

PRODUCT_COPY_FILES += \
    $(LOCAL_PATH)/init.rockchip.hasUMS.false.rc:root/init.$(TARGET_BOARD_HARDWARE).environment.rc
endif