zephyr: Add missing Kconfig dependencies for USB DFU

USB DFU depends on MULTITHREADING and STREAM_FLASH, and these
should be explicitly selected.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

boot/zephyr/Kconfig

@@ -419,6 +419,8 @@ config BOOT_USB_DFU_WAIT
 	select USB_DEVICE_STACK
 	select USB_DFU_CLASS
 	select IMG_MANAGER
+	select STREAM_FLASH
+	select MULTITHREADING
 	help
 	  If y, MCUboot waits for a prescribed duration of time to allow
 	  for USB DFU to be invoked. Please note DFU always updates the
@@ -429,6 +431,8 @@ config BOOT_USB_DFU_GPIO
 	select USB_DEVICE_STACK
 	select USB_DFU_CLASS
 	select IMG_MANAGER
+	select STREAM_FLASH
+	select MULTITHREADING
 	help
 	  If y, MCUboot uses GPIO to detect whether to invoke USB DFU.