roc-pc-*rk3399: use PREBOOT for default usb start

The boards are using CONFIG_USB_KEYBOARD=y, but usb doesn't get setup
automatically on boot since CONFIG_USE_PREBOOT=y is not present.

Author: samueldr

configs/roc-pc-mezzanine-rk3399_defconfig

@@ -23,6 +23,7 @@ CONFIG_SYS_LOAD_ADDR=0x800800
 CONFIG_PCI=y
 CONFIG_DEBUG_UART=y
 # CONFIG_ANDROID_BOOT_IMAGE is not set
+CONFIG_USE_PREBOOT=y
 CONFIG_DEFAULT_FDT_FILE="rockchip/rk3399-roc-pc-mezzanine.dtb"
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_MISC_INIT_R=y